Add some tests for RT #107254.

Author: pmichaud

t/qregex/rx_captures

@@ -60,4 +60,9 @@ $1=[ (.) (.) (.) ] (.)			abcdefg	<mob[5]: d @ 3>		perl5 numbered captures $1
 $<key>=<alpha>				12ab34		<mob<key>: a @ 2>	alias capture
 <key=alpha>				12ab34		<mob<key>: a @ 2>	alias capture
 
+:s $<w1>=(\w+) $<w2>=(\w+)? $<w3>=(\w+)?			one two three		<mob<w3>[0]: three @ 8>	RT #107254
+:s $<w1>=(\w+) $<w2>=(\w+)? $<w3>=(\w+)?			one two three		<mob<w2>[0]: two @ 4>	RT #107254
+:s $<w1>=(\w+) $<w2>=(\w+)? $<w3>=(\w+)?			one two			<mob<w2>[0]: two @ 4>	RT #107254
+:s :r $<w1>=(\w+) $<w2>=(\w+)? $<w3>=(\w+)?			one two			<mob<w2>[0]: two @ 4>	RT #107254
+
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