Add tests for backtracking into alternations

They're the bottom eight tests listed at
Raku/nqp#368
(The other ones listed there, for quantifiers,
seem to be already covered in S05-mass/rx.t)

This closes RT #130117 and #131973.

S05-metasyntax/longest-alternative.t

@@ -1,7 +1,7 @@
 use v6;
 use Test;
 
-plan 58;
+plan 62;
 
 #L<S05/Unchanged syntactic features/"While the syntax of | does not change">
 
@@ -476,4 +476,15 @@ is "abcde" ~~ / ab <![e]> cde | ab.. /, "abcde", 'negative lookahead does LTM pr
         'LTM with quantifier ** 1..2 followed by something else matches correctly';
 }
 
+
+# L<S05/Backtracking control>
+
+# RT #131973
+{
+    is 'ab' ~~ / [ab | a ] b /,       'ab', 'backtrack into |';
+    is 'ab' ~~ / [ab | a ]: b /,      Nil,  'don\'t backtrack into [ | ]:';
+    is 'ab' ~~ / :r [ab | a ] b /,    Nil,  'don\'t backtrack into | under :r';
+    is 'ab' ~~ / :r [ab | a ]:! b /,  'ab', 'backtrack into [ | ]:! despite :r';
+}
+
 # vim: ft=perl6 et

S05-metasyntax/sequential-alternation.t

@@ -1,6 +1,6 @@
 use v6;
 use Test;
-plan 10;
+plan 14;
 
 #L<S05/New metacharacters/"As with the disjunctions | and ||">
 
@@ -34,4 +34,15 @@ plan 10;
     is ~$/,  'xx', 'first ||@list alternative matches';
 }
 
+
+# L<S05/Backtracking control>
+
+# RT #130117 and #131973
+{
+    is 'ab' ~~ / [ab || a ] b /,       'ab', 'backtrack into ||';
+    is 'ab' ~~ / [ab || a ]: b /,      Nil,  'don\'t backtrack into [ || ]:';
+    is 'ab' ~~ / :r [ab || a ] b /,    Nil,  'don\'t backtrack into || under :r';
+    is 'ab' ~~ / :r [ab || a ]:! b /,  'ab', 'backtrack into [ || ]:! despite :r';
+}
+
 # vim: ft=perl6