ztest: shell: add shell support

- Support for listing both testcases and testsuites
- Support for running single suites or single test cases
- Support shuffling tests and repeating execution based on command line
  arguments.

For example, build with

west build -p  -b qemu_cortex_m3  tests/kernel/sleep   -t run -- \
-DCONFIG_ZTEST_SHUFFLE=y -DCONFIG_ZTEST_SHELL=y

Following commands are available:

uart:~$ ztest
ztest - Ztest commands
Subcommands:
  run-all          :Run all tests
  shuffle          :Shuffle tests
  list-testsuites  :List all test suites
  list-testcases   :List all test cases
  run-testsuite    :Run test suite
  run-testcase     :Run testcase

shuffle accepts two arguments --suite_iter and --case_iter which allows
repeated exercution of testcases or suites.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>