tests: port generic/093 to Linux

tests: port generic/093 to Linux

This IRIX-specific test mainly tested whether a file's capabilities are
cleared when it is written to.  Port the test to the Linux libcap tools
and update it to expect the Linux semantics which are a little simpler:
capabilities are always cleared even if the program is root (or has
CAP_FSETID).  The test also tests that chmod doesn't affect open file
descriptors; this is mostly unrelated, but keep it in for now.

[eguan: add _require_test_program rule for src/writemod]

Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>