JCLOUDS-457: Clean up for GlacierClientMockTest.

GlacierClientMockTest was growing uncontrollably. This commit
is an attempt to fix this problem.

The first measure taken moves the duplicated strings to constants
and the JSON response bodies to resource files. In addition, some
repeated code was moved to static classes, where they can be reused.

We also use @BeforeTest and @AfterTest to make instances of the
MockWebServer and shut them down for each test.