pitest#73 Force to disable junit5 parallel mode as PIT is not compati…

…ble with it

src/main/java/org/pitest/junit5/JUnit5TestPluginFactory.java

@@ -31,6 +31,7 @@ public Configuration createTestFrameworkConfiguration(TestGroupConfig config,
         ClassByteArraySource source, 
         Collection<String> excludedRunners,
         Collection<String> includedTestMethods) {
+        System.setProperty("junit.jupiter.execution.parallel.enabled", "false");
         return new JUnit5Configuration(config, includedTestMethods);
     }