Add tests for completion overwrite feature
Move accessibility tests to new completion test suite

The only functional change is that the previous `normalizeCompletion`
method is no longer applied. This shouldn't be necessary because a
string representation of a completion proposal should be clearly defined
and therefore not change between Scala or scalac versions.
Move some completion test cases to new test suite

- Simplify CompilerSupport in TextEditTests. It turned out that most
  behavior is already defined in SDTTestUtils. The new implementation
  is also more correct as it turned out while implementing tests. The
  previous implementation created a ScalaSourceFile directly, but by
  doing this it is not correctly wired together with the compilation
  unit. Creating a compilation unit directly creates also a
  ScalaSourceFile implicitly which can then be used later in the test
  implementation.
- Some test cases are moved from core.completion.CompletionTests to
  core.ui.completion.StandardCompletionTests. For this move the test
  cases where simplified a little bit. They no longer check the internal
  representation of a copmletion proposal but how the completion is
  displayed to users and how the document is changed after the
  completion is applied.
Move more completion test cases to new test suite

All test cases that belong together are moved to the new test suite.
This does not include ignored tests that test behavior which is not
implemented yet. It does also not include tests that mostly test a
different part of the completion behavior. It turned out that #1001014
can be closed because it is fixed.

While writing these tests the new test suite had to be extended further
times. Nevertheless, there are still tests that can still not be
implemented with the new test suite - therefore these tests aren't moved
either.

Re #1001014
Create new parameter completion tests

At the moment all tests are still disabled and therefore the old ones
are not yet removed. This is because the behavior is only insufficient
implemented.
Apply reviewer comments

- Change `IllegalArgumentException` to `ComparisonFailure`
- Move `caretOffset` from `TextEditTests` to `Operation`