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/secret/secret_test.go

@@ -25,6 +25,14 @@ var (
 	errNotImplementedInMock = errors.New("not implemented in mock")
 )
 
+// TestMain makes passphrase encryption in the tests cheap; see
+// ScryptWorkFactor.
+func TestMain(m *testing.M) {
+	ScryptWorkFactor = 1
+
+	os.Exit(m.Run())
+}
+
 // MockVault is a test implementation of the VaultInterface
 type MockVault struct {
 	name            string