httpserver: Added function to register directive at runtime (dev only)

This function should not be used outside of development. It destroys the
absolute ordering and guarantees of correctness. Multiple uses of it
may work fine, but maybe not if they overlap, causing non-deterministic
builds which is bad. However, this can be convenient when developing
a plugin by calling it from an init() function, since you don't have
to modify the Caddy source code just to try your plugin.

caddyhttp/httpserver/plugin_test.go

@@ -137,3 +137,23 @@ func TestInspectServerBlocksWithCustomDefaultPort(t *testing.T) {
 		t.Errorf("Expected the port on the address to be set, but got: %#v", addr)
 	}
 }
+
+func TestDirectivesList(t *testing.T) {
+	for i, dir1 := range directives {
+		if dir1 == "" {
+			t.Errorf("directives[%d]: empty directive name", i)
+			continue
+		}
+		if got, want := dir1, strings.ToLower(dir1); got != want {
+			t.Errorf("directives[%d]: %s should be lower-cased", i, dir1)
+			continue
+		}
+		for j := i + 1; j < len(directives); j++ {
+			dir2 := directives[j]
+			if dir1 == dir2 {
+				t.Errorf("directives[%d] (%s) is a duplicate of directives[%d] (%s)",
+					j, dir2, i, dir1)
+			}
+		}
+	}
+}