Add ability to immediately start ZK testing server in Curator extension

[sleberknight]

• Add two factory methods to CuratorTestingServerExtension that will
  start the Curator testing server immediately (at construction) instead
  of during the JUnit lifecycle (i.e. in beforeAll). These are named
  newStartingImmediately and one will start the testing server on a
  random port and the other takes a specific port to use.
• Before attempting to start the testing server, check that the port
  is available. Otherwise, when Curator's TestingServer attempts to
  start, it will never complete. The reason is that TestingServer starts
  the server in a new Thread and then blocks (without a timeout) until
  connected. We don't want this behavior in a test extension and instead
  should fail fast.
• Restructure existing test to verify nested test class behavior
  separate from tests having only top-level tests
• Add new test specifically to test the immediate start (necessary since
  we have to register the CuratorTestingServerExtension with immediate
  start explicitly)

Test restructuring:

Split the original CuratorTestingServerExtensionTest test into two
separate tests.

The first, CuratorTestingServerExtensionTest, contains only top-level
tests while the second, CuratorTestingServerExtensionNestedTest,
contains both top-level and nested test classes. These two contain
essentially the exact same tests, to ensure we are agnostic of how a
test is structured. Though as noted in extension's Javadoc, nested test
classes will cause us to use multiple testing servers (due to the way
the JUnit lifecycle works when using nested test classes).

Add CuratorTestingServerExtensionImmediateStartTest to explicitly test
the immediate start feature added in this commit.

Closes #239

[sonarqubecloud]

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