Description of problem:

Bug 726133 should be fixed soon so we will be able to remove case
runs from a test run. The caseruns should be implemented using the
Container class to allow both adding and removing test cases.

    TestRun(123).caseruns.add(TestCase(123))
    TestRun(123).caseruns.remove(TestCase(123))

Or, a new property testcases similar to that in TestPlan could be
added since iterating through test run test cases is also useful
and can be more handy than doing:

    if testcase in [caserun.testcase for caserun in testrun]

So the following could resolved this use case:

    TestRun(123).testcases.add(TestCase(123))
    TestRun(123).testcases.remove(TestCase(123))