tests: include current directory when running test Perl commands

tests: include current directory when running test Perl commands

Necessary to find generated files in the out-of-tree build directory.
E.g. `tests/configurehelp.pm`, for tests 1119 and 1167.

Before this patch macOS autotools builds were failing these two tests
due to falling back to the default preprocessor (`cpp`) instead of
the actual one configured. Then `cpp` failing to compile Apple SDK
headers referenced by curl headers.

Cherry-picked from #14097
Closes #14124