diff --git a/tests/helpers-testrepo.sh b/tests/helpers-testrepo.sh
--- a/tests/helpers-testrepo.sh
+++ b/tests/helpers-testrepo.sh
@@ -9,6 +9,13 @@
 # The mercurial source repository was typically orignally cloned with the
 # system mercurial installation, and may require extensions or settings from
 # the system installation.
+
+if [ -n $HGTESTEXTRAEXTENSIONS ]; then
+    for extension in $HGTESTEXTRAEXTENSIONS; do
+        extraoptions="$extraoptions --config extensions.$extension=!"
+    done
+fi
+
 syshg () {
     (
         syshgenv
@@ -48,6 +55,6 @@
     alias testrepohg=syshg
     alias testrepohgenv=syshgenv
 else
-    alias testrepohg=hg
+    alias testrepohg="hg $extraoptions"
     alias testrepohgenv=:
 fi
diff --git a/tests/hghave.py b/tests/hghave.py
--- a/tests/hghave.py
+++ b/tests/hghave.py
@@ -718,6 +718,10 @@
     except (ImportError, AttributeError):
         return False
 
+@check('extraextensions', 'whether tests are running with extra extensions')
+def has_extraextensions():
+    return 'HGTESTEXTRAEXTENSIONS' in os.environ
+
 def getrepofeatures():
     """Obtain set of repository features in use.
 
diff --git a/tests/run-tests.py b/tests/run-tests.py
--- a/tests/run-tests.py
+++ b/tests/run-tests.py
@@ -1070,6 +1070,17 @@
         env["HGENCODINGMODE"] = "strict"
         env['HGIPV6'] = str(int(self._useipv6))
 
+        extraextensions = []
+        for opt in self._extraconfigopts:
+            section, key = opt.encode('utf-8').split(b'.', 1)
+            if section != 'extensions':
+                continue
+            name = key.split(b'=', 1)[0]
+            extraextensions.append(name)
+
+        if extraextensions:
+            env['HGTESTEXTRAEXTENSIONS'] = b' '.join(extraextensions)
+
         # LOCALIP could be ::1 or 127.0.0.1. Useful for tests that require raw
         # IP addresses.
         env['LOCALIP'] = self._localip()
diff --git a/tests/test-basic.t b/tests/test-basic.t
--- a/tests/test-basic.t
+++ b/tests/test-basic.t
@@ -1,5 +1,6 @@
 Create a repository:
 
+#if no-extraextensions
   $ hg config
   devel.all-warnings=true
   devel.default-date=0 0
@@ -13,6 +14,8 @@
   web.address=localhost
   web\.ipv6=(?:True|False) (re)
   web.server-header=testing stub value
+#endif
+
   $ hg init t
   $ cd t
 
diff --git a/tests/test-debugextensions.t b/tests/test-debugextensions.t
--- a/tests/test-debugextensions.t
+++ b/tests/test-debugextensions.t
@@ -1,4 +1,6 @@
+#if no-extraextensions
   $ hg debugextensions
+#endif
 
   $ debugpath=`pwd`/extwithoutinfos.py
 
@@ -19,6 +21,10 @@
   > ext2 = `pwd`/extwithinfos.py
   > EOF
 
+  $ for extension in $HGTESTEXTRAEXTENSIONS; do
+  >     echo "$extension=!" >> $HGRCPATH
+  > done
+
   $ hg debugextensions
   ext1 (untested!)
   ext2 (3.2.1!)
diff --git a/tests/test-extension.t b/tests/test-extension.t
--- a/tests/test-extension.t
+++ b/tests/test-extension.t
@@ -867,9 +867,11 @@
 Show extensions:
 (note that mq force load strip, also checking it's not loaded twice)
 
+#if no-extraextensions
   $ hg debugextensions
   mq
   strip
+#endif
 
 For extensions, which name matches one of its commands, help
 message should ask '-v -e' to get list of built-in aliases
diff --git a/tests/test-globalopts.t b/tests/test-globalopts.t
--- a/tests/test-globalopts.t
+++ b/tests/test-globalopts.t
@@ -289,6 +289,8 @@
 
 Testing -h/--help:
 
+#if no-extraextensions
+
   $ hg -h
   Mercurial Distributed SCM
   
@@ -372,8 +374,6 @@
   
   (use 'hg help -v' to show built-in aliases and global options)
 
-
-
   $ hg --help
   Mercurial Distributed SCM
   
@@ -457,5 +457,7 @@
   
   (use 'hg help -v' to show built-in aliases and global options)
 
+#endif
+
 Not tested: --debugger
 
diff --git a/tests/test-help.t b/tests/test-help.t
--- a/tests/test-help.t
+++ b/tests/test-help.t
@@ -44,6 +44,10 @@
    summary       summarize working directory state
    update        update working directory (or switch revisions)
 
+Extra extensions will be printed in help output in a non-reliable order since
+the extension is unknown.
+#if no-extraextensions
+
   $ hg help
   Mercurial Distributed SCM
   
@@ -283,6 +287,8 @@
        win32mbcs     allow the use of MBCS paths with problematic encodings
        zeroconf      discover and advertise repositories on the local network
 
+#endif
+
 Verify that deprecated extensions are included if --verbose:
 
   $ hg -v help extensions | grep children
@@ -816,6 +822,8 @@
 
 Test that default list of commands omits extension commands
 
+#if no-extraextensions
+
   $ hg help
   Mercurial Distributed SCM
   
@@ -903,6 +911,7 @@
   
   (use 'hg help -v' to show built-in aliases and global options)
 
+#endif
 
 Test list of internal help commands