tests: Add ability to skip tests

tests/README.md

@@ -17,6 +17,8 @@ that is, `$i` and `$j` specify a range, 1-indexed, inclusive of both endpoints.
 highlighting should be observed.
 If `$todo` exists, the test point is marked as TODO (the failure of that test
 point will not fail the test), and `$todo` is used as the explanation.
+If a test sets `$skip_test` to a non-empty string, the test will be skipped
+with the provided string as the reason.
 
 **Note**: `$region_highlight` uses the same `"$i $j $style"` syntax but
 interprets the indexes differently.