This is in preparation for the tryjobs to run these tests. The LUCI harness will also be updated so that the tests to run are specified in the repo instead of the recipe.
Sample usage:
To run only the embedder_unittests in the engine with the profile variant, the command would be
```
./flutter/testing/run_tests.py --variant host_profile_unopt --type engine --filter embedder_unittests
``
To run only the geometry in Dart with the debug variant, the command would be
```
./flutter/testing/run_tests.py --variant host_debug_unopt --type dart --filter geometry_test
``
Without any argument, the behavior is identical to `run_tests.sh`.
In a subsequent patch, I will enable running unit-tests on Windows in the tryjobs. The lack of compatibility of the shell script on Windows made it so that we never ran any Windows unit-tests in the tryjobs.
