tests: enable strace-k.test only for --with-libunwind configurations

tests: enable strace-k.test only for --with-libunwind configurations

Also, since -k option does not support multiple personalities,
add strace-k.test to XFAIL_TESTS when running mpers tests.

* tests/Makefile.am (MPERS_NAME, LIBUNWIND_TESTS, XFAIL_TESTS_,
XFAIL_TESTS_m32, XFAIL_TESTS_mx32, XFAIL_TESTS): New variables.
[USE_LIBUNWIND] (LIBUNWIND_TESTS): Add strace-k.test.
(TESTS): Replace strace-k.test with $(LIBUNWIND_TESTS).
* bootstrap: Substitute MPERS_NAME variable in tests-*/Makefile.am.
* tests/strace-k.test: Do not check for -k option availability.