Remove libtestdriver1 with 'make clean'

It's a bit strange for tests/Makefile to clean up in library, but OTOH
it's also tests/Makefile that copies this file there.

Regardless, there was no place that cleaned up this file, and it needs to
be removed somewhere.

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>

tests/Makefile

@@ -203,6 +203,7 @@ ifndef WINDOWS
 	rm -f src/*.o src/drivers/*.o src/libmbed*
 	rm -f include/test/instrument_record_status.h
 	rm -rf libtestdriver1
+	rm -f ../library/libtestdriver1.a
 else
 	if exist *.c del /Q /F *.c
 	if exist *.exe del /Q /F *.exe