PSR2/UseDeclaration: move parse error test case to its own file

[biinari]

Description

Move parse error test to its own test case file.

Update comment to indicate that this is an intentional parse error. This was
implied by the original comment instructing that it must be the last line
in the test case file.

Suggested changelog entry

N/A

Related issues/external references

See #143

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
  • This change is only breaking for integrators, not for external standards or end-users.
• Documentation improvement

PR checklist

• I have checked there is no other PR open for the same change.
• I have read the Contribution Guidelines.
• I grant the project the right to include and distribute the code under the BSD-3-Clause license (and I have the right to grant these rights).
• I have added tests to cover my changes.
• I have verified that the code complies with the projects coding standards.
• [Required for new sniffs] I have added XML documentation for the sniff.

[jrfnl]

🎵 ...and another one bites the dust 🎵 Thanks @biinari!