gdb: create some automated tests with pytest

gem5 baremetal: use m5exit m5op in exit() so as to not force users to
apply a patch for almost all examples

baremetal/add.py

@@ -0,0 +1,9 @@
+def test(self):
+    self.sendline('tbreak main')
+    self.sendline('continue')
+    self.continue_to('op1')
+    assert self.get_int('i') == 1
+    self.continue_to('op2')
+    assert self.get_int('j') == 2
+    self.continue_to('result')
+    assert self.get_int('k') == 3