This PR creates a new test type for `run_tests.py` called `dart-host`.
The tests remaining under the `dart` type are tests that run in
`flutter_tester`. The `dart-host` tests run in the Dart CLI. This allows
`run_tests.py` to be simplified a bit, and while doing this I spotted a
couple of mistakes that were causing some tests not to run.
This PR also makes a couple of small style fixes to the build config
json files, converting tabs to spaces, and moving some "script" fields
before the "parameters" fields.
Using `engine_repo_tools`, provide a default `--target-variant` for the `clang_tidy` tool.
Mostly test code added below, the major change is that `--target-variant` now has a default other than `host_debug` if we're running inside of an engine root (which we ~always are). I didn't make any other changes (i.e. to the pre-commit hook) to keep this change smallish.
Also rewrote the `README.md` to represent the current state as best I could.
Partial work towards re-landing #44936.
Both the `clang_tidy` and `githooks` passage could benefit from being
able to automatically find the latest `compile_commands.json` output,
which means that some common code should exist in the `tools/`
directory.
This is a very minimal (but tested) library for doing exactly that.
