testing: avoid false warning

testing: avoid false warning

When FOLKS_BACKEND_PATH is set to the build dirs during testing, the
"content type of '%s' appears to be '%s' which looks suspicious. Have
you installed shared-mime-info?" warning gets triggered repeatedly,
because these dirs contain all kinds of files (executables, compiler
dependency files, etc.) which are not expected in the normal install
location.

This is annoying when running a test with "make <test>.gdb" because
gdb stops on a glib trace point for each warning. Therefore disable
the warning when FOLKS_BACKEND_PATH is set. Extending the whitelist
has the drawback that it might not catch all files that can appear
there in the future.