Fix broken FOSS build in import_test_coverage_spec.rb

97f102f4 introduced a fixture that is only present in EE. When the FOSS build attempt to run this test, it failed because the file did not exist.

To fix this, we only add this fixture under an EE build.

We also need to mute a FOSS-only failure with project.issues.events because that relation is only tested in the EE-specific fixture.

Closes #197298 (closed)