Run tests/common's selftests in one binary: 21 helper selftests are compiled into and re-run by every one of ~104 includers #1957

[agent] Filed by the scheduled architecture audit (audit-cli-tests). Register: register comment.

Kind: refactor. Register ID: T58.

Problem

The shared test-support modules carry their own #[test] selftests, which are deliberately left ungated:

• common/mod.rs mod oracle_selftests L421: 9 tests;
• common/cache_env.rs mod cache_env_selftests L365: 10 tests;
• common/jvm_env.rs mod jvm_env_selftests L112: 2 tests.

104 test files include #[path = "common/mod.rs"] mod common;, so each of those binaries compiles and runs all of them. cargo test --test cli_apply_silent reports 27 tests for 7 owned. That adds up to roughly 2,000 to 2,500 redundant test runs per cargo test. Some binaries include cache_env.rs both through common/mod.rs and directly (14 found, e.g. e2e_redirect_bun_build), so those run the cache_env set twice.

The justification comment is false. mod.rs L416-L420 says "Integration-test crates do NOT have cfg(test) set", and the same claim appears in cache_env.rs L361-L364 and bundler_e2e.rs L167-L169. cargo test --test self_update_failures_e2e -- --list lists update_fixture::fixture_selftests, which is #[cfg(test)], so cfg(test) is set (executed). Gating on cfg(test) would not help, though: every includer is a test crate.

Proposed change

• Move the three selftest modules into one dedicated binary, e.g. tests/common_selftests.rs, which includes common/mod.rs, cache_env.rs and jvm_env.rs and owns the tests. Delete the modules from the shared files.
• Delete the three false cfg(test) comments.
• Drop the direct #[path = "common/cache_env.rs"] includes from binaries that already get it through common (re-export cache_env via common::cache_env).

Size

4 support files plus 1 new test file, and about 14 #[path] lines. About −40 / +15 lines, with 21 tests moved verbatim.

Acceptance criteria

• cargo test -p socket-patch-cli --test cli_apply_silent -- --list lists only that suite's own tests.
• cargo test -p socket-patch-cli --test common_selftests runs all 21 selftests, unchanged and green.
• No other test changes behavior.