t/test-lib.sh: export PERL_PATH for use in scripts

t/test-lib.sh: export PERL_PATH for use in scripts

Most notably, t4031 creates a small shell script that invokes perl
and we want to use "$PERL_PATH" to name the version of Perl suitable
for our use, read from GIT-BUILD-OPTS.  The test would fail when it
is directly run in t/ directory from the shell or "make" is run in t/
directory.

This problem was hidden from "make test" run in the top-level
directory, because its Makefile exports PERL_PATH.

Signed-off-by: Junio C Hamano <redacted>