Core Rule Set tests: load each set of settings once (closes #252)

Tests now share one Core Rule Set for each set of settings, loaded once
through sync.OnceValues in shared(). Exclusions-file tests and
TestCorazaWritesNoFile still load their own: loading is what they check.

Loading was not most of the time: the WordPress and Drupal recorded
requests, inspected one after another, were. Each recorded request now
runs as its own parallel subtest, with the same request and outcome.

Model: opus-5-5

internal/coreruleset/coreruleset_test.go

@@ -36,7 +36,8 @@ var loaded = struct {
 }{loaders: map[string]func() (*coreruleset.CoreRuleSet, error){}}
 
 // shared returns the Core Rule Set with params, loaded once for every
-// test that asks for the same settings.
+// test that asks for the same settings. Loading takes most of the time
+// the tests take.
 func shared(t *testing.T, params coreruleset.Params) *coreruleset.CoreRuleSet {
 	t.Helper()