relesetools: Enable releasetools_test in presubmit.

About half of the testcases rely on external tools (i.e. the ones in
`otatools.zip`, which are external to releasetools module, but still
built by Android). It's WAI as releasetools scripts are mostly for
gluing purpose.

However, the current support in Soong doesn't allow packing the helper
modules as part of the built releasetools_test. This CL adds a decorator
that allows declaring external dependencies in testcases, which will be
skipped while running in presubmit. It doesn't affect local invocation
of `atest releasetools_test`.

Fixes: 112080715
Test: `atest releasetools_test`
Test: TreeHugger; check that releasetools_test is invoked (and test
      passes).
Change-Id: I8fdeb6549023cf5ddeb79d610c7c37cf9f13d3cc

tools/releasetools/test_utils.py

@@ -31,6 +31,18 @@ import common
 # Some test runner doesn't like outputs from stderr.
 logging.basicConfig(stream=sys.stdout)
 
+# Use ANDROID_BUILD_TOP as an indicator to tell if the needed tools (e.g.
+# avbtool, mke2fs) are available while running the tests. Not having the var or
+# having empty string means we can't run the tests that require external tools.
+EXTERNAL_TOOLS_UNAVAILABLE = not os.environ.get("ANDROID_BUILD_TOP")
+
+
+def SkipIfExternalToolsUnavailable():
+  """Decorator function that allows skipping tests per tools availability."""
+  if EXTERNAL_TOOLS_UNAVAILABLE:
+    return unittest.skip('External tools unavailable')
+  return lambda func: func
+
 
 def get_testdata_dir():
   """Returns the testdata dir, in relative to the script dir."""