Remove the PhasedTestManager.changeExceptionMessage method

As of Java16 setAccessible is no longer alllowed. Therefore we need to find a new way to manipulate the exception message in the PhasedTesting

Examples:
PhasedTestManagerTests.testChangeExceptio_NullPointer

1. Create exception of same type
2. Attach cause
3. Add additional message

Start by locating PhasedTestManager.changeExceptionMessage and the PhasedTestManagerTests.testChangeExceptio_NullPointer test. Confirm the current behavior, then determine how to preserve the exception type, cause, and additional message without setAccessible; done means the method is removed and the test behavior remains covered.

Written by the indexing model from the issue text.