[Linux] add GTest printer for FlValue

Improves test failure messages when matching FlValue arguments.

For example, before:

  Expected arg #2: has setting ("platformBrightness", 0x32f0980)
           Actual: 0x7f81ec005990, 0x32f0900

And after:

  Expected arg #2: has setting ("platformBrightness", dark)
           Actual: 0x27bded0, {textScaleFactor: 0.0, alwaysUse24HourFormat: false, platformBrightness: light}

shell/platform/linux/testing/fl_test.cc

@@ -62,3 +62,8 @@ FlEngine* make_mock_engine_with_project(FlDartProject* project) {
 
   return static_cast<FlEngine*>(g_object_ref(engine));
 }
+
+void PrintTo(FlValue* v, std::ostream* os) {
+  g_autofree gchar* s = fl_value_to_string(v);
+  *os << s;
+}

shell/platform/linux/testing/fl_test.h

@@ -6,9 +6,11 @@
 #define FLUTTER_SHELL_PLATFORM_LINUX_FL_TEST_H_
 
 #include "flutter/shell/platform/linux/public/flutter_linux/fl_engine.h"
+#include "flutter/shell/platform/linux/public/flutter_linux/fl_value.h"
 
 #include <glib.h>
 #include <stdint.h>
+#include <ostream>
 
 G_BEGIN_DECLS
 
@@ -27,6 +29,9 @@ FlEngine* make_mock_engine();
 // platform messages.
 FlEngine* make_mock_engine_with_project(FlDartProject* project);
 
+// GTest printer for FlValue.
+void PrintTo(FlValue* v, std::ostream* os);
+
 G_END_DECLS
 
 #endif  // FLUTTER_SHELL_PLATFORM_LINUX_FL_TEST_H_