make gdb test automation even awesomer

baremetal/arch/arm/regs.S

@@ -0,0 +1,8 @@
+/* See the aarch64 version. */
+.global main
+main:
+    mov r0, #1
+    /* test-gdb-r0 */
+    mov r1, #2
+    /* test-gdb-r1 */
+    bx lr

baremetal/arch/arm/regs.py

@@ -0,0 +1,8 @@
+def test(self):
+    self.sendline('tbreak main')
+    self.sendline('continue')
+    self.continue_to('r0')
+    self.sendline('set $r0 = 3')
+    self.continue_to('r1')
+    assert self.get_int('$r0') == 3
+    assert self.get_int('$r1') == 2