Single literal regexp value testcase for querier

It's common to see queries like bar=~"foo" from machine generated
queries in the fronted. These are not evaluated as regexps, but are a
single-value-set, i.e. and equality matchings instead.

This is just a testcase for a single-value case.

Signed-off-by: Oleg Zaytsev <mail@olegzaytsev.com>

tsdb/querier_test.go

@@ -1513,6 +1513,13 @@ func TestFindSetMatches(t *testing.T) {
 		pattern string
 		exp     []string
 	}{
+		// Single value, coming from a `bar=~"foo"` selector.
+		{
+			pattern: "^(?:foo)$",
+			exp: []string{
+				"foo",
+			},
+		},
 		// Simple sets.
 		{
 			pattern: "^(?:foo|bar|baz)$",