Fix test failures on certain machines (#571)

On certain machines (one of mine) the outcastTest consistently fails due
to the following error:

java.lang.NoClassDefFoundError: Could not initialize class
google.registry.persistence.transaction.JpaTestRules$JpaIntegrationTestRule

If I reduce maxparallelForks to 3 it consistently passes. This issue was
mentioned here:

https://discuss.gradle.org/t/junit-test-fails-with-noclassdeffounderror-only-when-maxparallelforks-1/6047

But this post was 8 years old and no solution was identified.

core/build.gradle

@@ -709,7 +709,10 @@ task outcastTest(type: FilteringTest) {
   tests = outcastTestPatterns
 
   // Sets the maximum number of test executors that may exist at the same time.
-  maxParallelForks 5
+  // Note that this number appears to contribute to NoClassDefFoundError
+  // exceptions on certain machines and distros. The root cause is unclear.
+  // Try reducing this number if you experience similar problems.
+  maxParallelForks 3
 }
 
 // Whitebox test verifying that RegistryTool can be instantiated. Note the