Fix parallel test execution on Linux

Apparently the Qt backend does not like being used in tests that are run
in multiple threads, so force the fluent style instead.

tests/driver/interpreter/interpreter.rs

@@ -14,6 +14,7 @@ pub fn test(testcase: &test_driver_lib::TestCase) -> Result<(), Box<dyn Error>>
         .collect::<Vec<_>>();
     let mut compiler = sixtyfps_interpreter::ComponentCompiler::default();
     compiler.set_include_paths(include_paths);
+    compiler.set_style(String::from("fluent")); // force to fluent style as Qt does not like multi-threaded test execution
 
     let component =
         spin_on::spin_on(compiler.build_from_source(source, testcase.absolute_path.clone()));