testing: kselftest_harness: add filtering and enumerating tests

As the number of test cases and length of execution grows it's
useful to select only a subset of tests. In TLS for instance we
have a matrix of variants for different crypto protocols and
during development mostly care about testing a handful.
This is quicker and makes reading output easier.

This patch adds argument parsing to kselftest_harness.

It supports a couple of ways to filter things, I could not come
up with one way which will cover all cases.

The first and simplest switch is -r which takes the name of
a test to run (can be specified multiple times). For example:

  $ ./my_test -r some.test.name -r some.other.name

will run tests some.test.name and some.other.name (where "some"
is the fixture, "test" and "other" and "name is the test.)

Then there is a handful of group filtering options. f/v/t for
filtering by fixture/variant/test. They have both positive
(match -> run) and negative versions (match -> skip).
If user specifies any positive option we assume the default
is not to run the tests. If only negative options are set
we assume the tests are supposed to be run by default.

  Usage: ./tools/testing/selftests/net/tls [-h|-l] [-t|-T|-v|-V|-f|-F|-r name]
	-h       print help
	-l       list all tests

	-t name  include test
	-T name  exclude test
	-v name  include variant
	-V name  exclude variant
	-f name  include fixture
	-F name  exclude fixture
	-r name  run specified test

  Test filter options can be specified multiple times. The filtering stops
  at the first match. For example to include all tests from variant 'bla'
  but not test 'foo' specify '-T foo -v bla'.

Here we can request for example all tests from fixture "foo" to run:

 ./my_test -f foo

or to skip variants var1 and var2:

 ./my_test -V var1 -V var2

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>