Do not skip system health smoke test if --also-run-disabled-tests flag is passed

System health test are skipped dynamically, so unlike other Telemetry tests which are disabled
with @dectorators.Skip(..), the logic to not skip these tests have be done by the sites that invoke self.skipTest(..)

Bug: 893618
Change-Id: I2c2869c5ec75613e797bc68d4c546b698f949869
Reviewed-on: https://chromium-review.googlesource.com/c/1287099Reviewed-by: Caleb Rouleau <crouleau@chromium.org>
Commit-Queue: Ned Nguyen <nednguyen@google.com>
Cr-Commit-Position: refs/heads/master@{#600569}

tools/perf/benchmarks/system_health_smoke_test.py

@@ -117,7 +117,7 @@ def _GenerateSmokeTestCase(benchmark_class, story_to_smoke_test):
     if possible_browser is None:
       self.skipTest('Cannot find the browser to run the test.')
 
-    if self.id() in _DISABLED_TESTS:
+    if self.id() in _DISABLED_TESTS and not options.run_disabled_tests:
       self.skipTest('Test is explicitly disabled')
 
     single_page_benchmark = SinglePageBenchmark()