From @marco-c:

Adding some static rules

• If a test is modified, run the manifest that includes it
• If a support file is modified, run the manifest that includes it
• If a file in a directory is modified, run the manifest that is in the same directory (e.g. if I modify https://searchfox.org/firefox-main/source/dom/battery/BatteryManager.cpp, we should run https://searchfox.org/firefox-main/source/dom/battery/test/chrome.toml and https://searchfox.org/firefox-main/source/dom/battery/test/mochitest.toml)