tests: split p4fastimport-import-lfs.t into normal LFS vs SQLite
p4fastimport has the ability to bypass the normal LFS upload mechanisms, and
that's what the original testcase actually tested.
We also want to ensure that normal LFS works with p4fastimport too, so we need
a testcase for that.
Differential Revision: https://phab.mercurial-scm.org/D1306