Added skipped count to JUnit TestClassResult.

The skipped count is calculated as the number of results whose result type is `skipped`.
Updated JUnitXmlWriter to output skipped tests in the same format as Ant and Maven.

 - Added `skipped` attribute to `testsuite` with a value of the number of tests that were skipped.
 - When a test is skipped:
	- The test case's element name is now `testcase`.
	- An empty sub-element of `skipped` is added.
Modified JUnitTestClassExecutionResult to support the changes made in JUnitXmlResultWriter.

 - Overloaded `assertTestCount` with a 4 integer function that asserts the skip count.
 - Changed assert methods to filter on the existence(or not) of a `skipped` sub-element.
 - Added an assert to the checking method to check that there is a skipped annotation and that is not empty.
 - Removed findIgnoredTests as it is no longer required.
Modified JUnit XML report specification test.

 - Changed test count assert to include skipped count.
 - Modified all XML blocks to include the `skipped` attribute.
 - Modified the skipped test case to use the new `testcase` element name and `skipped` sub-element.