test(app): run launcher matrix tests in parallel

[umputun]

the three launcher tests ran strictly serially and took 121s of the app package's 139s, leaving about 26s of headroom under the 180s race budget. That budget already shaped test design: the stderr expectations were folded into an existing matrix instead of getting their own pass, to avoid crossing it.

the 139 leaf subtests are independent. Each gets its own TMPDIR, passes an explicit environment to its subprocess through mergeEnv, and runs its launcher in its own process group via Setpgid. Marking them parallel is safe on all three counts.

result on a 20-core mac

	before	after
app package, race	139-154s	62-72s
headroom under the 180s budget	~26s	~110s

the win comes from saturating idle cores rather than overlapping idle waits. Individual subtests get slower under contention, so the work is process-spawn bound, not sleep bound.

two details worth recording:

• t.Parallel() has to precede the windows skip guard, or tparallel still reports the parent as serial.
• a parallel parent is safe next to the t.Setenv tests in config_test.go and termbg_test.go. Go pauses every parallel test until all serial ones finish, so they never overlap and no env var can leak into a launcher subprocess.

verification: 8 consecutive green runs of the three tests, golangci-lint clean, and the full suite passing at the real -timeout=180s.

[Copilot]

🟢 Approval recommended

The change is limited to test parallelization, and the test harness isolates per-subtest temp/state via per-test TMPDIR + explicit subprocess env, avoiding shared-process mutations.

Pull request overview

This PR reduces CI/runtime for the app package launcher-matrix tests by running independent subtests in parallel, improving headroom under the race-test timeout budget.

Changes:

• Mark TestShellLaunchersPreserveAnnotationExitCode, TestHerdrSignalPaneOwnership, and TestHerdrPaneOverlayOptIn as parallel.
• Mark the launcher-matrix subtests (t.Run(...)) inside those tests as parallel to enable concurrent execution across launcher/backends/cases.

File summaries

File	Description
app/plugin_exit_code_test.go	Adds t.Parallel() to the three launcher matrix tests and their subtests to run the matrix concurrently and cut wall-clock time.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 0
• Review effort level: Lite

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