tests: poison $HOME and $XDG_RUNTIME_DIR env variables

To attempt to catch unit tests which accidentally create files in $HOME,
or $XDG_RUNTIME_DIR, poison these env vars by pointing them to
directories which don't exist. This should give easier to debug test
failures. For example:

$ VIR_TEST_DEBUG=1 ./qemuhotplugtest
Could not initialize HostdevManager - operation failed: Failed to create state dir '/bad-test-used-env-xdg-runtime-dir/libvirt/hostdevmgr'
Reviewed-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NAndrea Bolognani <abologna@redhat.com>
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>

tests/testutils.c

@@ -767,6 +767,9 @@ int virTestMain(int argc,
         preloads[npreloads] = NULL;
     }
 
+    g_setenv("HOME", "/bad-test-used-env-home", TRUE);
+    g_setenv("XDG_RUNTIME_DIR", "/bad-test-used-env-xdg-runtime-dir", TRUE);
+
     va_start(ap, func);
     while ((lib = va_arg(ap, const char *))) {
         if (!virFileIsExecutable(lib)) {