[3.14] gh-145177: Add emscripten run --test, uses test args from config.toml (GH-146160) (#146186)

This allows us to change the test arguments from the python repo rather
than having to change buildmaster-config.
(cherry picked from commit 6b5511d66bab0754d1d959cfe98947c536bf4d82)

Co-authored-by: Hood Chatham <roberthoodchatham@gmail.com>
Co-authored-by: Russell Keith-Magee <russell@keith-magee.com>