Make the tests fast under the race detector (closes #120)

Deriving keys from passphrases with scrypt, slow on purpose, took most
of the test time under -race. secret.ScryptWorkFactor, when not zero,
replaces age's work factor when a passphrase encrypts; the tests of
internal/secret, internal/vault and internal/cli set it to 1 in
TestMain, and the program never sets it. TestGetCommandOutputsToStdout
checks that the built binary's passphrase unlocker names age's 18.

TestRemovalAsksWithoutHoldingLock and TestFailedCommandReleasesLock
time the in-memory lock all tests share, so they no longer run in
parallel. TestConcurrentAddsKeepEveryVersion and
TestGetCommandOutputsToStdout time nothing and now do.

The script/cibuild comment no longer says tests are skipped without
its memlock ulimit.

Model: opus-5-5

internal/vault/vault_test.go

@@ -3,6 +3,7 @@ package vault_test
 import (
 	"bytes"
 	"errors"
+	"os"
 	"path/filepath"
 	"slices"
 	"testing"
@@ -28,6 +29,14 @@ const (
 	testPassphrase = "test-passphrase"
 )
 
+// TestMain makes passphrase encryption in the tests cheap; see
+// secret.ScryptWorkFactor.
+func TestMain(m *testing.M) {
+	secret.ScryptWorkFactor = 1
+
+	os.Exit(m.Run())
+}
+
 // testMnemonicBuffer returns testMnemonic in a locked buffer that is
 // destroyed when the test ends.
 func testMnemonicBuffer(t *testing.T) *memguard.LockedBuffer {