[ENH] add negative test cases in `TestAllMetrics`.

[PranavBhatP]

Is your feature request related to a problem? Please describe.
In the current version of the test_all_metrics.py we are only testing for positive test cases in the context of metrics and their functionality. It would be nice to have negative test cases to ensure that the metrics return the right errors in case of an incorrect inputs to the metrics API and other related cases.

[fkiraly]

interesting idea. In sktime, negative test cases are often not part of the test suite but of a test_base module, since the expectation is that the base layer handles negative cases.

However, it is perhaps better coverge to do this for all metrics - hence in TestAllMetrics.

[fkiraly]

is this a good first issue? In case we add more concrete instructions where to add what? Or did you want to take this?

[PranavBhatP]

Yes I think this can be a good first issue. I will try adding a few lines more about what should exactly be done.