running multiple tests in simpletest

the simpletest_run_tests() contains a check on the constant variable TEST_RUNNING, and will only run a test if the variable is not set. As this is a constant it can only be set once and, in theory at least, prevents multiple calls to simpletest_run_tests from running in the same page load.

The attached patch converts the constant to a global variable to allow the variable to be reset after the test has completed.