Changed test to use logical naming, so that no domain knowledge is
requried, and so that the point of each test is easier to comprehend.
Introduced a prodAndDevRequestsForContentPluginsAreEmpty() method to
make tests easier to comprehend.
Finished updating the tests so that they are specified in a logical
manner with no domain information, plus added loads of missing tests
that must have got lost when the XML content plug-in was ported from the
old code base.
Added a test for anonymous element merging, and it seems like we've
always supported this all along.
Merge pull request #755 from BladeRunnerJS/anonymous-element-merging

allowing merging of elements without IDs in XML bundler