Add a section about testing to the Polonius book

Author: Albin Stjerna

book/src/SUMMARY.md

@@ -7,4 +7,5 @@
 - [Rules](./rules.md)
   - [Atoms](./rules/atoms.md)
   - [Initialization](./rules/initialization.md)
+- [Testing Polonius](./testing.md)
 - [See also](./see_also.md)

book/src/testing.md

@@ -0,0 +1,16 @@
+# Testing Polonius
+
+## Rust UI Tests with Polonius Compare Mode
+
+There is a mode of the Rust test suite that compares Polonius' output to the
+current NLL one. You can invoke it by using `--compare-mode polonius`. For
+example, the following will run the UI tests:
+
+```
+$ ./x.py test -i --stage 1 --compare-mode polonius src/test/ui
+```
+
+## Polonius' Own Unit Test
+
+(Not yet written, but this section should describe how to use `polonius-parser`
+to generate input for unit tests.)