Add options and filename kwargs to testing wrappers

[ggueret]

Summary

• Adds options= and filename= keyword arguments to all eight wrappers in rude.testing (check_source, check_codes, fix_source, assert_no_errors, assert_error, assert_error_count, assert_fix, assert_no_fix).
• options=: instantiates the rule, then calls .configure(options) before checking. Plugin tests no longer need to bypass rude.testing to test configurable rules.
• filename=: forwards to FileContext.path, useful for rules that branch on the path.

Context

Stacked on #35. Diff above main includes the utils helpers from that PR.

Test plan

• check_source forwards options to rule.configure(); skips the call when options=None
• check_source forwards filename to FileContext.path
• fix_source forwards both
• The 6 other wrappers (check_codes, assert_no_errors, assert_error, assert_error_count, assert_fix, assert_no_fix) accept and forward both kwargs

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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