[LNT] Support TEST_SUITE_SUBDIRS for test_suite

[ychen]

Summary

Comes handy when you want to exclude (or add) some tests.

[ychen]

--cmake-define works just fine.

lnt/tests/test_suite.py

def _configure(self, path, extra_cmake_defs=[], execute=True):
             # Ensure that no flags get added based on build type when the user
             # explicitly specifies flags to use.
             for build_type in cmake_build_types:
                 defs['CMAKE_CXX_FLAGS_'+build_type] = ""
 
         if self.opts.run_under:
             defs['TEST_SUITE_RUN_UNDER'] = \
                 self._unix_quote_args(self.opts.run_under)
+        if self.opts.subdirs:
+            defs['TEST_SUITE_SUBDIRS'] = \
+                self._unix_quote_args(self.opts.subdirs)
         if self.opts.benchmarking_only:
             defs['TEST_SUITE_BENCHMARKING_ONLY'] = 'ON'
         if self.opts.only_compile:
             defs['TEST_SUITE_RUN_BENCHMARKS'] = 'Off'
         if self.opts.use_perf in ('time', 'all'):
             defs['TEST_SUITE_USE_PERF'] = 'ON'
         if self.opts.test_suite_externals:
             defs['TEST_SUITE_EXTERNALS_DIR'] = self.opts.test_suite_externals
@click.option("--test-size", "test_size",
               type=click.Choice(['small', 'regular', 'large']),
               default='regular', help="The size of test inputs to use")
 @click.option("--benchmarking-only", "benchmarking_only", is_flag=True,
               help="Benchmarking-only mode. Disable unit tests and "
                    "other flaky or short-running tests")
 @click.option("--only-test", "only_test", metavar="PATH",
               type=click.UNPROCESSED, default=None,
               help="Only run tests under PATH")
[e-mail]("--subdirs", "subdirs", metavar="String",
+              type=click.UNPROCESSED, default=None,
+              help="Semicolon separated list of directories with "
+                   "CMakeLists.txt to include")
 # Test Execution
 @click.option("--only-compile", "only_compile",
               help="Don't run the tests, just compile them.", is_flag=True)
 @click.option("-j", "--threads", "threads",
               help="Number of testing (and optionally build) "
               "threads", type=int, default=1, metavar="N")
 @click.option("--build-threads", "build_threads",
               help="Number of compilation threads, defaults to --threads",