test harness: Fix off-by-one discrepancy between observed and expected.

Fixes issue #195.

tests/README.md

@@ -5,6 +5,13 @@ Utility scripts for testing zsh-syntax-highlighting highlighters.
 
 The tests expect the highlighter directory to contain a `test-data` directory with test data files. See the [main highlighter](../highlighters/main/test-data) for examples.
 
+Each test should define the array parameter `$expected_region_highlight`.
+The value of that parameter is a list of `"$i $j $style"` strings.
+Each string specifies the highlighting that `$BUFFER[$i,$j]` should have;
+that is, `$i` and `$j` specify a range, 1-indexed, inclusive of both endpoints.
+
+_Note_: `$region_highlight` uses the same `"$i $j $style"` syntax but interprets the indexes differently.
+
 
 highlighting test
 -----------------