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.

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/cli/integration_test.go

@@ -52,8 +52,12 @@ func runSecretWithStdin(stdin string, env map[string]string, args ...string) (st
 	return cli.ExecuteCommandInProcess(args, stdin, env)
 }
 
-// TestMain runs before all tests and ensures the binary is built
+// TestMain runs before all tests and ensures the binary is built. It also
+// makes passphrase encryption in the tests cheap (see
+// secret.ScryptWorkFactor); the binary keeps age's work factor.
 func TestMain(m *testing.M) {
+	secret.ScryptWorkFactor = 1
+
 	// Get the current working directory
 	wd, err := os.Getwd()
 	if err != nil {