Add test utility class for getting RecordedRequests from an OkHttp MockWebServer

[sleberknight]

The test utilities should (initially) provide a way to:

• get a "required" RecordedRequest, or throw an IllegalStateException without blocking indefinitely like the MockWebServer#takeRequest() method does
• get a RecordedRequest or return an empty Optional if one is not available
• get a RecordedRequest or return null if one is not available
• assert that there are no more RecordedRequest available (using AssertJ assertions)

None of these should make you catch InterruptedException like the MockWebServer#takeRequest methods do. Instead, wrap with an unchecked exception or catch and return null, Optional.empty(), throw an exception, etc. Since these are test utilities, ignoring InterruptedException isn't a big deal.

Maybe also provide a few methods with specific timeouts like I did in MockWebServerExtensions.kt, for example takeRequestWith1SecTimeout.

Also consider adding similar methods to the urlWithoutTrailingSlashAsString methods in MockWebServerExtensions.kt. We can't make them extensions as in Kotlin, but they're still useful.