[!119] Test now fails if calculated rate is NaN.

Merge branch 'convergencetest/NaN' into 'master'

ref:quality/dune-testtools Before, the test was passed if "calculated rate" = NaN because the if-condition for failure was not satisfied.

See merge request quality/dune-testtools!119

(cherry picked from commit f51a124a)

4ee3e827 Test now fails if calculated rate is NaN. 2526a12b Explicit check for inf and nan. Also inverts logic to check for success and fails else.