selftests: lib.mk: add SKIP handling and test suite name to EMIT_TESTS

EMIT_TESTS which is the common function that implements run_tests target,
treats all non-zero return codes from tests as failures. When tests are
skipped with non-zero return code, because of unmet dependencies and/or
unsupported configuration, it reports them as failed. This will lead to
too many false negatives even on the tests that couldn't be run.

EMIT_TESTS is changed to test for SKIP=4 return from tests to enable
the framework for individual tests to return special SKIP code.

Tests will be changed as needed to report SKIP instead FAIL/PASS when
they get skipped.

Currently just the test name is printed in the RUN_TESTS output. For
example, when raw_skew sub-test from timers tests in run, the output
shows just raw_skew. Include main test name when printing sub-test
results.

In addition, remove duplicate strings for printing common information with
a new for the test header information.

With this change run_kelftest.sh output for breakpoints test will be:

TAP version 13
Running tests in breakpoints
========================================
selftests: breakpoints: step_after_suspend_test
not ok 1..1 selftests: breakpoints: step_after_suspend_test [SKIP]
selftests: breakpoints: breakpoint_test
ok 1..2 selftests: breakpoints: breakpoint_test [PASS]

Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>