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/drupal_test.go

@@ -21,18 +21,22 @@ func TestOrdinaryDrupalRequestsPass(t *testing.T) {
 	defaults, bodies := atDefaults(t), readingBodies(t)
 
 	for _, recorded := range recordedRequests(t, "drupal-requests.txt") {
-		r := request{recorded.Method, recorded.Target, recorded.Headers}
+		t.Run(recorded.Method+" "+recorded.Target, func(t *testing.T) {
+			t.Parallel()
 
-		got, _ := inspectBody(t, defaults, r, recorded.Body)
-		if got.Score >= drupalThreshold {
-			t.Errorf("%s %s refused: %+v", r.method, r.target, got)
-		}
+			r := request{recorded.Method, recorded.Target, recorded.Headers}
 
-		got, _ = inspectBody(t, bodies, r, recorded.Body)
-		if got.Score >= drupalThreshold {
-			t.Errorf("%s %s %.200q refused while bodies are read: %+v",
-				r.method, r.target, recorded.Body, got)
-		}
+			got, _ := inspectBody(t, defaults, r, recorded.Body)
+			if got.Score >= drupalThreshold {
+				t.Errorf("%s %s refused: %+v", r.method, r.target, got)
+			}
+
+			got, _ = inspectBody(t, bodies, r, recorded.Body)
+			if got.Score >= drupalThreshold {
+				t.Errorf("%s %s %.200q refused while bodies are read: %+v",
+					r.method, r.target, recorded.Body, got)
+			}
+		})
 	}
 }