[swift-ide-test] Use same code completion extraction logic for batch code completion and single code completion

If the test case hase a code completion token of the form `#^TOKEN?check=CHECK^#` from batch code completion, this allows us to run `swift-ide-test` with `--code-completion-token TOKEN` instead of `--code-completion-token 'TOKEN?check=CHECK'`, which is more convenient.

Author: ahoppen