[test] Add paths to chrome and firefox to the environment when present
[test] Add paths to chrome and firefox to the environment when present

This adds a CHROME_PATH or FIREFOX_PATH environment variable to the
test's evnironment which contains a path to the browser's executable.
This is only set if the --chrome or --firefox argument was passed to
test.py.

https://github.com/dart-lang/sdk/issues/42417

Change-Id: I711de6ab91b8ffbbe72c7fb1530b68c4189a4b8a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/153209
Commit-Queue: Alexander Thomas <29642742b6693024c89de8232f2e2542cf7eedf7@google.com>
Reviewed-by: William Hesse <a821cddceae7dc400f272e3cb1a72f400f9fed6d@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk