test: skip one test case when running on coverage

test: skip one test case when running on coverage

Since 90fa161b5ba29d58953e9f08ddca49121b51efe6, --bind= or Bind=
settings for coverage directory does not work with managed mode:
```
[  158.105361] systemd-nspawn[3718]: Failed to open tree and set mount attributes: Operation not permitted
[  158.105364] systemd-nspawn[3718]: Failed to clone /coverage: Operation not permitted
[  158.118655] systemd-nspawn[3707]: (sd-namespace) failed with exit status 1.
```
Let's tentatively skip the test case when running on coverage.

Workaround for #38163.