gpu: nvgpu: posix: Multithreading for unit tests

Add a -j argument to enable running unit tests on several
threads. Also adds signal handling to prevent a fatal
error in one thread from killing the whole unit test
framework.

JIRA NVGPU-1043

Change-Id: I891a547640cd005a50ffa5c06367ed46c54de012
Signed-off-by: Nicolas Benech <nbenech@nvidia.com>
Reviewed-on: https://git-master.nvidia.com/r/1847740
Reviewed-by: svc-misra-checker <svc-misra-checker@nvidia.com>
GVS: Gerrit_Virtual_Submit
Reviewed-by: Alex Waterman <alexw@nvidia.com>
Reviewed-by: mobile promotions <svcmobile_promotions@nvidia.com>
Tested-by: mobile promotions <svcmobile_promotions@nvidia.com>

userspace/include/unit/unit.h

@@ -23,6 +23,8 @@
 #ifndef __UNIT_UNIT_H__
 #define __UNIT_UNIT_H__
 
+#include <pthread.h>
+
 struct gk20a;
 
 struct unit_module;
@@ -84,6 +86,8 @@ struct unit_module {
 	 */
 	void				*lib_handle;
 	struct unit_fw			*fw;
+
+	pthread_t thread;
 };
 
 /*