ms_driver: Do the existance check with the path

Change the existance test to use the full path and not just the
binary name.

Change-Id: Ibe28514df03efb7c302ac30411761a8d1a6dcf5d

src/osmo_gsm_tester/ms_driver.py

@@ -93,7 +93,7 @@ class MsDriver(log.Origin):
         def check_and_return_binary(name):
             """Checks the binary exists and returns the path."""
             binary = inst.child('bin', name)
-            if not os.path.isfile(name):
+            if not os.path.isfile(binary):
                 raise RuntimeError('Binary missing: %r' % binary)
             return binary