interp: add glob regression test from master

The original issue mvdan#347 only affected v2, not v3, but it's still useful
to have this regression test for the future. After all, we only happened
to fix this bug in v3 as part of a correctness refactor, as we hadn't
found the bug until recently.

interp/interp_test.go

@@ -2108,6 +2108,10 @@ set +o pipefail
 		"mkdir x-d1 x-d2; touch x-f; echo ././x-*/// | sed -e 's@\\\\@/@g'",
 		"././x-d1/ ././x-d2/\n",
 	},
+	{
+		"mkdir -p x-d1/a x-d2/b; touch x-f; echo x-*/* | sed -e 's@\\\\@/@g'",
+		"x-d1/a x-d2/b\n",
+	},
 	{
 		"mkdir x-d; touch x-f; test -d $PWD/x-*/",
 		"",