build: Enable maintainer flags when compiling tests
build: Enable maintainer flags when compiling tests

The tests should be compiled under the same flags as the rest of the
library.

robtaylor/json-glib-gvariant,ebassi/json-glib,oerdnj/json-glib,GNOME/json-glib,robtaylor/json-glib-gvariant,oerdnj/json-glib,GNOME/json-glib,oerdnj/json-glib,Distrotech/json-glib,oerdnj/json-glib,ebassi/json-glib,brauliobo/json-glib,Distrotech/json-glib,ebassi/json-glib,brauliobo/json-glib,frida/json-glib,brauliobo/json-glib,Distrotech/json-glib,frida/json-glib