Hold runMu in internal/cli tests that log outside a CLI run (closes #210)

A CLI run under runMu points the process-global logger at its own
buffers, so a line another test logged outside runMu could land in that
run's output. TestCheckQuietShowsFailedFiles caught one: a "loaded
manifest" line in the stderr of a passing check -q run.

The test helpers that scan or load a manifest now hold runMu
themselves, and so do the remaining direct loads and scans, the
recordEntry call and the retrying download. Two dotfile tests use
manifestPaths instead of loading the manifest themselves.
TestRootVerbosityFlags ran on its own only to avoid those lines, so it
now runs in parallel.

Judgement call: a downloadFile call that gets its answer on the first
request does not log, so those calls are left as they are.

Model: opus-5-5

internal/cli/errmsg_test.go

@@ -54,6 +54,11 @@ func runLocked(fn func() error) error {
 func unsignedChecker(t *testing.T) *mfer.Checker {
 	t.Helper()
 
+	// The scan and the manifest load log to the process-global logger, so
+	// serialize with the CLI runs.
+	runMu.Lock()
+	defer runMu.Unlock()
+
 	fs := afero.NewMemMapFs()
 	require.NoError(t, fs.MkdirAll("/d", 0o755))
 	require.NoError(t, afero.WriteFile(fs, "/d/f.txt", []byte("hi"), 0o644))
@@ -183,6 +188,11 @@ func signedManifest(t *testing.T, files map[string][]byte) []byte {
 func signedChecker(t *testing.T, manifest []byte) *mfer.Checker {
 	t.Helper()
 
+	// Loading a manifest logs to the process-global logger, so serialize
+	// with the CLI runs.
+	runMu.Lock()
+	defer runMu.Unlock()
+
 	fs := afero.NewMemMapFs()
 	require.NoError(t, afero.WriteFile(fs, "/index.mf", manifest, 0o644))