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.

tests/test-highlighting.zsh

@@ -97,10 +97,12 @@ run_test_internal() {
   echo "# ${1:t:r}"
 
   # Load the data and prepare checking it.
-  local BUFFER CURSOR MARK PENDING PREBUFFER REGION_ACTIVE WIDGET
+  local BUFFER CURSOR MARK PENDING PREBUFFER REGION_ACTIVE WIDGET skip_test
   local -a expected_region_highlight region_highlight
   . "$srcdir"/"$1"
 
+  (( $#skip_test )) && { print -r -- "1..0 # SKIP $skip_test"; return; }
+
   # Check the data declares $PREBUFFER or $BUFFER.
   [[ -z $PREBUFFER && -z $BUFFER ]] && { echo >&2 "Bail out! Either 'PREBUFFER' or 'BUFFER' must be declared and non-blank"; return 1; }
   # Check the data declares $expected_region_highlight.