Fix unexpected test failure in test_strwidth()

[danielbachhuber]

In #155, we marked a test skipped 217cb51 because it was unexpectedly failing.

We should track down why it was failing, and restore the test.

Done is:

• $this->markTestSkipped() is removed from the test.

[schlessera]

It passed successfully on my local machine, so my current guess would be that it is either related to the environments locale/encoding or to a missing or differently configured PHP extension.
Once we find out which it is, we still need to figure out whether it is something we need to adapt the tests for, or whether the tests currently are indeed detecting an edge case we failed to take into account.