make(1): improve test for undefined subexpression in :C modifier

In the previous test output, it was unnecessarily hard to see what
happens because of the many empty strings.  Enclosing the capturing
groups in parentheses helps against this.

usr.bin/make/unit-tests/moderrs.exp

@@ -96,7 +96,7 @@ make: No match for subexpression \2
 make: No match for subexpression \1
 make: No match for subexpression \2
 make: No match for subexpression \1
-+ + + 3 5 8 3+ +1 34
+()+() ()+() ()+() 3 5 8 (3)+() ()+(1) 34
 mod-ts-parse:
 112358132134
 15152535558513521534

usr.bin/make/unit-tests/moderrs.mk

@@ -1,4 +1,4 @@
-# $Id: moderrs.mk,v 1.13 2020/08/09 15:03:25 rillig Exp $
+# $Id: moderrs.mk,v 1.14 2020/08/09 15:15:29 rillig Exp $
 #
 # various modifier error tests
 
@@ -140,7 +140,7 @@ mod-regex-delimiter:
 mod-regex-undefined-subexpression:
 	@echo $@:
 	@echo ${FIB:C,1(.*),one\1,}		# all ok
-	@echo ${FIB:C,1(.*)|2(.*),\1+\2,}	# no match for subexpression
+	@echo ${FIB:C,1(.*)|2(.*),(\1)+(\2),:Q}	# no match for subexpression
 
 mod-ts-parse:
 	@echo $@: