Fix tests in `AccessibilityTests`

Because the test suite now watches compilation errors, the tests did no
longer succeed (they were broken since the beginning). After the
insertion of a package name everything should work fine now.
Revert "Mark some completion tests as flaky."

This reverts commit 8c66015c68004bd028cfcc42a44c9707eb76c999.

The reason why the tests are flaky is that all files are created in the
default package, which can lead to name clashes. The real fix is to move
the files to unique packages.
Fix tests in `CompletionOverwriteTests`

The package declaration helps to avoid name clashes.