protobuf tests are fragile

The protobuf tests are using some testing patterns that make them fragile. I've only looked at tst_protobuf_basictypes_qml::qtprotobufBasicTest, but other tests likely do similar stuff.

• Test initialization done in test_1initialization instead of initTestCase
• Init data modified by other test functions in place
  • e.g sfixed32Msg.testFieldFixedInt32 modified by test_basicTypesOperations_data