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/helpers_test.go

@@ -5,11 +5,9 @@ package runner
 
 import (
 	"context"
-	"net"
 	"os/exec"
 	"runtime"
 	"testing"
-	"time"
 
 	"gitea.com/gitea/runner/act/container"
 
@@ -42,18 +40,6 @@ func requireDocker(t *testing.T) {
 	}
 }
 
-// requireNetwork skips the test unless github.com is reachable. A few tests exercise behaviour
-// that inherently needs the network (force-pulling an image, resolving a remote short-sha ref);
-// gating lets the rest of the suite run offline without these failing.
-func requireNetwork(t *testing.T) {
-	t.Helper()
-	conn, err := net.DialTimeout("tcp", "github.com:443", 3*time.Second)
-	if err != nil {
-		t.Skipf("skipping: network unavailable: %v", err)
-	}
-	_ = conn.Close()
-}
-
 // requireHostTools skips the test unless every named executable is on PATH. Used by the
 // self-hosted (host environment) suite, which runs steps directly on the host.
 func requireHostTools(t *testing.T, tools ...string) {