Fix ext/VMS-Filespec/t/filespec.t after move in 2d11a7e9678a88.

Moving it caused it to fail because it expected test.pl to exist
in the directory it's being run from, but that isn't the case
anymore.  We can now just depend on the test infrastructure to
control @INC and don't need any special handling of library
directories.
Add non-XS extensions to known_extensions in configure.com.

Follow-up to f7b3892b9e45c6994.

For now we're not re-sorting known_extensions after combining the
XS and non-XS extensions into one list.  It would be pretty arduous
to do that in DCL and probably doesn't matter.

We special-case VMS::Filespec because (at least on VMS) it's an XS
extension so clearly doesn't belong in nonxs_ext.  Technically it's
static because it's XS bits live in vms/vms.c, but it's not clear
that it belongs in static_ext as those would normally be things
that require an extra link step (if such static linking even
works, which we don't know as it hasn't been tested in a long
time.)