Tests: inline TestUtils.swift in NIOHTTP1Tests to avoid broken symlink on non-symlink filesystems (#3426)

On filesystems without symlink support for Windows, the symlinked test
helper at is materialized as a plain text file containing the relative
path, which breaks compilation with errors like:
```
expressions are not allowed at the top level
cannot find 'assertNoThrowWithValue' in scope
```
This change inlines the contents, restoring the intended shared test
utilities for the NIOHTTP1 test target. This helps Windows compile the
Test target

Author: zamderax