tests: Add lib- prefix to all mocks

In preparation libtool "-module" flag removal, add lib prefix to all
mock shared objects.

While at it, introduce VIR_TEST_MOCK macros that makes path out of mock
name to be used with VIR_TEST_PRELOAD or VIR_TEST_MAIN_PRELOAD.  That,
hopefully, improves readability, reduces line length and allows to
tailor VIR_TEST_MOCK for specific platform if it has shared library
suffix different from ".so".

Signed-off-by: Roman Bolshakov <r.bolshakov@yadro.com>

tests/testutils.h

@@ -148,6 +148,10 @@ int virTestMain(int argc,
         return virTestMain(argc, argv, func, __VA_ARGS__, NULL); \
     }
 
+#define MOCK_EXT ".so"
+
+#define VIR_TEST_MOCK(mock) (abs_builddir "/.libs/lib" mock "mock" MOCK_EXT)
+
 virCapsPtr virTestGenericCapsInit(void);
 int virTestCapsBuildNUMATopology(virCapsPtr caps,
                                  int seq);