Speed up the vaultik and database package tests (closes #235)

In the Dockerfile test phase, internal/vaultik spent 23.6s of 31.9s in
36 tests run one at a time. 24 of them were serial only because they
call log.Initialize; they now call it before t.Parallel(), so the
logger is replaced before any parallel test runs. The 12 still serial
change the umask, TMPDIR, os.Stderr or time.Local.

TestLargeDatasets took 8.4s of the 9.8s internal/database run by
committing each of its 1,500 inserts on its own; it now makes them in
one transaction. TestDedupOnlySnapshotRestores gives its second backup
its own snapshot name instead of sleeping 1.1s for a new snapshot ID.

Model: opus-5-5

internal/vaultik/restore_interrupt_test.go

@@ -164,10 +164,9 @@ func scratchEntries(t *testing.T, dir string) []string {
 // restore while a blob download is in progress. The download fails only
 // because of the cancel, so Restore must return context.Canceled without
 // reporting the file that needs the blob as failed.
-//
-//nolint:paralleltest // installs the global logger via log.Initialize
 func TestRestoreSkipErrorsCancelDuringBlobDownload(t *testing.T) {
 	log.Initialize(log.Config{})
+	t.Parallel()
 
 	fs := afero.NewOsFs()
 	tempDir := t.TempDir()