improve grcov '--ignore' globs

[cylewitruk-stacks]

Improve grcov --ignore glob patterns:

• Use ** instead of redundant **/* (globset docs say ** matches zero or more path segments, including files).
• Use brace expansion test{,s} to consolidate test/tests matching into single patterns (current pattern misses stackslib/chainstate/stacks/index/test/**, e.g. in perf: add small root-node LRU cache for MARF storage connections stacks-core#6944).
• Add separate **/*_test{,s}.rs pattern to catch suffixed test files (e.g. foo_test.rs/bar_tests.rs) without over-matching (the previous **/*tests.rs glob could match potential non-test files like contest.rs).

[Copilot]

Pull request overview

Updates the grcov ignore globs used by the composite GitHub Action to more precisely exclude test-related paths/files (and avoid over-matching), improving coverage signal quality for the Stacks codebases that use this action.

Changes:

• Simplifies directory ignore globs from **/* forms to **.
• Attempts to consolidate test/tests ignores using test{,s} brace syntax.
• Replaces a broad *tests.rs ignore with more targeted *_test{,s}.rs-style patterns.

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[dhaney-stacks]

lgtm, 🚢 it!