Get rid of all remaining JUnit 4 usages except in prober & proxy (#731)

* Get rid of all remaining JUnit 4 usages except in prober & proxy subprojects

Caveat: Test suites aren't yet implemented in JUnit 5 so we still use the ones
from JUnit 5 in the core subproject.

* Fix some build errors

buildSrc/src/main/java/google/registry/gradle/plugin/GcsPluginUtils.java

@@ -57,6 +57,10 @@ final class GcsPluginUtils {
     return file.toPath().toAbsolutePath().normalize();
   }
 
+  static Path toNormalizedPath(Path file) {
+    return file.toAbsolutePath().normalize();
+  }
+
   static String getContentType(String fileName) {
     return EXTENSION_TO_CONTENT_TYPE.getOrDefault(
         Files.getFileExtension(fileName), DEFAULT_CONTENT_TYPE);