tests: Make references to global symbols indirect in test drivers

A library has to be built with -flat_namespace to get all references to
global symbols indirected. That can also be achieved with two-level
namespace interposition but we're not using explicit symbol
interposition since it's more verbose and requires massive changes to
the mocks.

This provides a way to interpose a mock for virQEMUCapsProbeHostCPU from
qemucpumock and fixes domaincapstest on macOS.
Signed-off-by: NRoman Bolshakov <r.bolshakov@yadro.com>

tests/Makefile.am

@@ -56,6 +56,9 @@ DRIVERLIB_LDFLAGS = \
 	-avoid-version \
 	-rpath /evil/libtool/hack/to/force/shared/lib/creation \
 	$(MINGW_EXTRA_LDFLAGS)
+if WITH_MACOS
+DRIVERLIB_LDFLAGS += -Wl,-flat_namespace
+endif WITH_MACOS
 
 PROBES_O =
 if WITH_DTRACE_PROBES