Split MpMetricTest

[jmartisk]

MpMetricTest in the REST test suite has become too bloated, too many tests in one test case, and to make it worse, the order of tests is important (some tests depend on the outcome from previous tests).
This makes it very hard

• to maintain and extend the test case
• to run just a single test
• to exclude some problematic tests when needed for troubleshooting purposes
• to read the reports if some tests failed

It would be better to split it into multiple test cases.