test: cover LLGo directive comments

[cpunion]

Summary

• add LLGo runtime tests for //go:linkname, //llgo:link, // llgo:link, and //llgo:skip
• add a build-tagged test/go fixture containing //llgo:skipall so the directive spelling is covered without changing default test package codegen
• keep this PR test-only; it does not include go:nointerface support or compiler refactoring

Tests

• go test ./test/go
• LLGO_BUILD_CACHE=off go run ./cmd/llgo test ./test/go -run 'TestLLGoDirective'

Notes

• The //llgo:skip test is a link-time regression check: if the skipped function is compiled, it references a deliberately missing C symbol and the package fails to link.
• The //llgo:skipall fixture is behind an explicit build tag because enabling skipall in the normal test package skips package codegen, including the package init symbol expected by the generated test main.

[gemini-code-assist]

Code Review

This pull request introduces test cases for llgo compiler directives, including link, skip, and skipall. Feedback indicates that the skip directive test should be adjusted to prevent dead code elimination from masking failures and that the skipall fixture requires specific build tags to be properly exercised during testing.

[fennoai]

Review Summary

This is a small, focused test-only PR that adds runtime tests for LLGo directive comments (//go:linkname, //llgo:link, //llgo:skip, //llgo:skipall). The //go:linkname and //llgo:link coverage is solid and provides direct, asserted verification. A few quality/consistency points worth addressing:

• Missing license headers: Other test files in test/go/ (e.g., binop_test.go, cgo_malloc_test.go) carry the standard Apache 2.0 header; both new files omit it.
• Legacy // +build lines: No other file in the directory uses the old-style constraint; let go fmt manage these or drop them for consistency.
• Import grouping: The standalone import _ "unsafe" should be folded into the grouped block — this matches the convention in promoted_offsetof_test.go, string_slice_base_test.go, etc.
• Passive skip tests: TestLLGoDirectiveSkip is empty and the skipall fixture has no driver — both rely entirely on link-time failure as evidence. This is a valid technique but warrants explanatory comments and ideally a more active assertion. The fixture file in particular has no obvious harness invoking it with the directive_skipall_fixture tag.
• Anchor types: directiveSkipAnchor and directiveSkipAllAnchor are unused; a one-line comment explaining they exist as doc-comment anchors for the directive would prevent future "dead code" cleanup.

No security or performance concerns.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.48%. Comparing base (b4d9167) to head (5442f6a).
⚠️ Report is 11 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1834   +/-   ##
=======================================
  Coverage   88.48%   88.48%           
=======================================
  Files          50       50           
  Lines       14418    14418           
=======================================
  Hits        12758    12758           
  Misses       1440     1440           
  Partials      220      220           

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