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/stdout_stderr_test.go

@@ -15,11 +15,11 @@ import (
 // TestGetCommandOutputsToStdout tests that 'secret get' outputs the secret
 // value to stdout, not stderr
 func TestGetCommandOutputsToStdout(t *testing.T) {
-	// Create a temporary directory for our vault
-	tempDir := t.TempDir()
+	t.Parallel()
 
-	// Set environment variables for the test
-	t.Setenv(secret.EnvStateDir, tempDir)
+	// Create a temporary directory for our vault; each command is given it
+	// in its environment
+	tempDir := t.TempDir()
 
 	// Find the secret binary path
 	wd, err := filepath.Abs("../..")