testTaskDistribution: fix random failures with test_04c_startTestSuiteOrderWithManyTestNodes

test_04c_startTestSuiteOrderWithManyTestNodes is using same test results
names ("test suite 1", "test suite 2", ...) as some previous tests (
test_04b_startTestSuiteOrder ) so it's safer to start the test by
cancelling all previous test runs and renaming them.

/reviewed-on !959