Fix flaky test (open-telemetry#13358)

Author: laurit

javaagent-tooling/src/main/java/io/opentelemetry/javaagent/tooling/config/ConfigurationFile.java

@@ -39,6 +39,11 @@ static Map<String, String> getProperties() {
     return configFileContents;
   }
 
+  // visible for tests
+  static void resetForTest() {
+    configFileContents = null;
+  }
+
   // visible for tests
   static Map<String, String> loadConfigFile() {
     // Reading from system property first and from env after

javaagent-tooling/src/test/java/io/opentelemetry/javaagent/tooling/config/ConfigurationPropertiesSupplierTest.java

@@ -32,6 +32,7 @@ class ConfigurationPropertiesSupplierTest {
   @AfterAll
   static void cleanUp() {
     GlobalOpenTelemetry.resetForTest();
+    ConfigurationFile.resetForTest();
   }
 
   // regression for https://github.com/open-telemetry/opentelemetry-java-instrumentation/issues/6696