Add notes about running the UI test suite

src/tests/running.md

@@ -158,6 +158,21 @@ Note that incremental compilation will use more disk space than usual.
 If disk space is a concern for you, you might want to check the size
 of the `build` directory from time to time.
 
+## Running tests with different "compare modes"
+
+UI tests may have different output depending on certain "modes" that
+the compiler is in. For example, when in "non-lexical lifetimes" (NLL)
+mode a test `foo.rs` will first look for expected output in
+`foo.nll.stderr`, falling back to the usual `foo.stderr` if not found.
+To run the UI test suite in NLL mode, one would use the following:
+
+```bash
+./x.py test src/test/ui --compare-mode=nll
+```
+
+Other examples of compare-modes are "noopt", "migrate", and
+[revisions](./adding.html#revisions).
+
 ## Running tests manually
 
 Sometimes it's easier and faster to just run the test by hand. Most tests are