test_bootloader: Fix sporadic failure

This test is broken in multiple ways. Either:

1) Execution of diff races against the named temporary file being
removed when the context manager is left, resulting in no output from
diff and the test "passes" (diff exits with status 2).

2) diff generates output and the call to assertRegexpMatches() fails
   because it was renamed in Python 3.2.

Fix this by ensuring diff finishes before leaving the context manager,
asserting the exact diff expected, and checking the return code.

Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>

Author: rosslagerwall

tests/test_bootloader.py

@@ -17,13 +17,20 @@ def test_grub2(self):
             proc = subprocess.Popen(["diff", "tests/data/grub.cfg", temp.name],
                                     stdout = subprocess.PIPE,
                                     universal_newlines=True)
-        assert proc.stdout
-        for line in proc.stdout:
-            # pylint: disable-next=deprecated-method
-            self.assertRegexpMatches(line, r"^(5a6,13$|>)")
 
-        proc.stdout.close()
-        proc.wait()
+            self.assertEqual(proc.stdout.read(), '''5a6,13
+> if [ -s $prefix/grubenv ]; then
+> 	load_env
+> fi
+> 
+> if [ -n "$override_entry" ]; then
+> 	set default=$override_entry
+> fi
+> 
+''')
+            proc.stdout.close()
+            proc.wait()
+            self.assertEqual(proc.returncode, 1)
 
     def test_no_multiboot(self):
         # A module2 line without a multiboot2 line is an error