fileserver: Fix newly-introduced failing test on Linux (#3625)

* fileserver: First attempt to fix failing test on Linux

I think I updated the wrong test case before

* Make new test function

I guess what we really are trying to test is the case insensitivity of
firstSplit. So a new test function is better for that.

modules/caddyhttp/fileserver/matcher_test.go

@@ -71,11 +71,6 @@ func TestPHPFileMatcher(t *testing.T) {
 			expectedPath: "/foo.php.php/index.php",
 			matched:      true,
 		},
-		{
-			path:         "/foo.php.PHP/index.php",
-			expectedPath: "/foo.php.PHP/index.php",
-			matched:      true,
-		},
 		{
 			// See https://github.com/caddyserver/caddy/issues/3623
 			path:         "/%E2%C3",
@@ -115,3 +110,12 @@ func TestPHPFileMatcher(t *testing.T) {
 		}
 	}
 }
+
+func TestFirstSplit(t *testing.T) {
+	m := MatchFile{SplitPath: []string{".php"}}
+	actual := m.firstSplit("index.PHP/somewhere")
+	expected := "index.PHP"
+	if actual != expected {
+		t.Errorf("Expected %s but got %s", expected, actual)
+	}
+}