Use mj_parseXMLString instead of a test-specific LoadFromModelString.

This removes the need for a custom mjResourceProvider, but does require
reimplementing part of mj_loadXML in the fixture.

PiperOrigin-RevId: 854170820
Change-Id: Ib455cc5cdb5138213eb7e996dc32ac156f22a859

src/xml/xml_global.h

@@ -20,7 +20,7 @@
 
 #include <mujoco/mujoco.h>
 
-void SetGlobalXmlSpec(mjSpec* spec = nullptr);
+MJAPI void SetGlobalXmlSpec(mjSpec* spec = nullptr);
 
 std::string GetGlobalXmlSpec(const mjModel* m, char* error, int error_sz);