awk/tests: xfail inf-nan-torture on riscv64

The fix for this is being tracked upstream here:
https://github.com/onetrueawk/awk/issues/269

While here, just cd into $SRCDIR while executing tests,
since the test engine isolates every testcase's working
directory. This ensures that the xfail actually applies
to the next command.

Reviewed by: mhorne
MFC after: 3 days
Sponsored by: The FreeBSD Foundation