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/prune_count_test.go

@@ -14,10 +14,9 @@ import (
 // the discarded-error bug: getTableCount for a table its query cannot
 // resolve must not silently become 0. A count that could not be read is
 // reported as unknown, which a reader can tell apart from an empty table.
-//
-//nolint:paralleltest // installs the global logger via log.Initialize
 func TestTableCountForReportSurfacesReadFailure(t *testing.T) {
 	log.Initialize(log.Config{})
+	t.Parallel()
 
 	ctx := context.Background()