diffguard-testkit/fixtures.rs: unnecessary hashes on raw string literals

[EffortlessSteven]

clippy::raw_string_literal: these raw strings don't contain so the extra hashes are unnecessary.

Locations (all in ):

• Line 374:
• Line 386:
• Line 398:
• Line 417:
• Line 434:
• Line 449:
• Line 469:
• Line 484:
• Line 499:
• Line 518:
• Line 550:
• Line 562:

Fix: change to → (remove the delimiters).

Note: only filed here since most are in test/fixture code. If the 6 in the main lib are already tracked separately, no need to dupe.