Check number of coupling iterations with HT test

[BenjaminRodenberg]

#115 added the HT tutorials as a test. However, one thing is still missing to replace the current systemtests: We should also check the number of required coupling iterations to be able to detect regressions.

Two important questions:

• Do we actually want to replace the systemtest? See discussion under https://github.com/precice/systemtests/issues/267
• If yes, how much additional complexity do we get here? How much code duplication does this introduce? Currently the systemtests repository bundles certain functionality for several tests (result comparison, docker). Do we need this functionality here?