test: speed up (#1121)

Full suite 178s → 96s, and a run's log 12MB → 2KB. `act/runner` was 177s of the 178s, serialised behind one docker daemon.

- Its fixtures now run in parallel, bounded by a slot count instead of `go test -parallel`, with a per-test container name prefix and a pinned `MaxParallel`.
- Fixtures asserting a job failure leaked their container and network (`AutoRemove` was at the act-CLI default), filling the daemon's address pool over time.
- Replaced sleeps used as synchronisation in the parallel-executor and cache-handler tests.
- Dropped duplicate coverage: two files re-testing `NewParallelExecutor`, a test asserting on its own semaphore, and `TestDockerActionForcePullForceRebuild`, whose config `runTest` discarded.
- `fmt-check`/`security-check` move from `make test` to a `checks` target; `security-check` no longer installs `xgo` and `gxz`.
- Test flags follow gitea: `GOTEST_FLAGS ?= -race -timeout 20m -parallel 8`, with `-cover`/`-coverprofile` left in the target. Dropped `-v`, since a failing package still prints its full output without it.

Coverage unchanged at 73.4%.

Reviewed-on: https://gitea.com/gitea/runner/pulls/1121
Reviewed-by: bircni <bircni@icloud.com>
Co-authored-by: silverwind <me@silverwind.io>

act/runner/job_executor_test.go

@@ -33,6 +33,7 @@ import (
 )
 
 func TestJobExecutor(t *testing.T) {
+	t.Parallel()
 	// Dryrun only checks syntax/planning; all cases resolve locally, so this runs offline.
 	tables := []TestJobFileInfo{
 		{workdir, "uses-and-run-in-one-step", "push", "Invalid run/uses syntax for job:test step:Test", platforms, secrets},
@@ -46,6 +47,7 @@ func TestJobExecutor(t *testing.T) {
 	ctx := common.WithDryrun(context.Background(), true)
 	for _, table := range tables {
 		t.Run(table.workflowPath, func(t *testing.T) {
+			t.Parallel()
 			table.runTest(ctx, t, &Config{})
 		})
 	}