XPCShell tests test Thunderbird's components, without opening the user interface. Firefox also runs this type of test, and much of the information about Firefox's XPCShell tests also apply to Thunderbird.
To run an XPCShell test, or a directory of them, use mach:
mach must point to mach. The path argument is always relative to the mozilla-central root, so include comm/ at the start.
If more than one test runs, just a summary of the results will be displayed. You can pass --verbose to get the full output if necessary.
You may notice a long pause when running tests for the first time after building. Mach is reindexing the tests.
Mochitest
Like XPCShell tests, mochitests are a type of test used on Firefox. The main difference is that mochitest runs with the full UI, in the context of the main mail window.
To run a mochitest, or a directory of them, use mach: