cache_impl_test.go: fix failing test with ipv6

A newly-added test in envoyproxy#137 checks the exact text of an error message which
seems to vary when the network is tcp4 vs tcp6. This change relaxes the
assertion to look for "connection refused" in a panic without making
assumptions about what an IP address looks like.

Example failure:

--- FAIL: TestNewClientImpl (0.00s)
    --- FAIL: TestNewClientImpl/connection_refused (0.00s)
        cache_impl_test.go:442:
                Error Trace:    cache_impl_test.go:442
                Error:          func (assert.PanicTestFunc)(0x1724110) should panic with error message: "dial tcp 127.0.0.1:12345: connect: connection refused"
                                        Panic value:    "dial tcp [::1]:12345: connect: connection refused"
                                        Panic stack:    goroutine 27 [running]:

The testify assert package doesn't seem to support inexact matching on error messages, so the code gets a bit uglier than before.

Signed-off-by: David Weitzman <dweitzman@pinterest.com>

test/redis/cache_impl_test.go

@@ -430,6 +430,17 @@ func mustNewRedisServer() *miniredis.Miniredis {
 	return srv
 }
 
+func expectPanicError(t *testing.T, f assert.PanicTestFunc) (result error) {
+	t.Helper()
+	defer func() {
+		panicResult := recover()
+		assert.NotNil(t, panicResult, "Expected a panic")
+		result = panicResult.(error)
+	}()
+	f()
+	return
+}
+
 func TestNewClientImpl(t *testing.T) {
 	redisAuth := "123"
 	statsStore := stats.NewStore(stats.NewNullSink(), false)
@@ -439,11 +450,10 @@ func TestNewClientImpl(t *testing.T) {
 	}
 
 	t.Run("connection refused", func(t *testing.T) {
-		assert.PanicsWithError(t, "dial tcp 127.0.0.1:12345: connect: connection refused", func() {
-			// It's possible there is a redis server listening on 6379 in ci environment, so
-			// use a random port.
-			mkRedisClient("", "localhost:12345")
-		})
+		// It's possible there is a redis server listening on 6379 in ci environment, so
+		// use a random port.
+		panicErr := expectPanicError(t, func() { mkRedisClient("", "localhost:12345") })
+		assert.Contains(t, panicErr.Error(), "connection refused")
 	})
 
 	t.Run("ok", func(t *testing.T) {