The ImprotControlTest rule should govern which imports aren't (and are) allowed in test files. However, for many tests, this rule is suppressed, so the configuration is not adhered to, leading to classes that aren't supposed to be used in tests.

This PR aims to address this issue.
It removes these suppressions, and where needed, replaces them with finer-grained "allow" entries in import-control-test.xml.

This PR addresses the following concerns:

Usage of reflection

• As per the guidelines of All reflection-based test should be reviewed and changed (or whitelisted) #11123, tests that use reflection to either introspect on the internal (private) state of an object, or execute some method/constructor that should be private, were rewritten to use the utilities from TestUtil that provide this functionality, so there's a clear (or at least, clearer) separation of responsibilities.
• Tests that use reflection to assert the existence/order/values of Checkstyle classes were explicitly called out with <allow> entries in import-control-test.xml. There may be room for additional improvement here in the future, but it is out of scope for this PR.
• Tests that test Checkstyle APIs that have java.lang.reflect parameters or return values were explicitly allowed to import those classes with <allow> entries.

Usage of JUnit

• assertThrows assertions were replaced with TestUtil. getExpectedThrowable, as per Wrap assertThrows with getExpectedThrowable method. #11343
• All JUnit assertions other than assertDoesNotThrow were written with Truth assertions instead of JUnit assertions.
• The import-control-test.xml syntax for disallowing JUnit and Hamcrest assertions was fixed.

Fixes #17910