tests/avocado: partially revert unmasking of replay_linux tests

It seems we were premature in declaring replay_linux.py fixed. The
x86_64 image still seems to hang occasionally. I've raised a new bug
to cover it:

    https://gitlab.com/qemu-project/qemu/-/issues/2094

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>

tests/avocado/replay_kernel.py

@@ -98,10 +98,13 @@ def test_i386_pc(self):
 
         self.run_rr(kernel_path, kernel_command_line, console_pattern, shift=5)
 
+    # See https://gitlab.com/qemu-project/qemu/-/issues/2094
+    @skipUnless(os.getenv('QEMU_TEST_FLAKY_TESTS'), 'Test sometimes gets stuck')
     def test_x86_64_pc(self):
         """
         :avocado: tags=arch:x86_64
         :avocado: tags=machine:pc
+        :avocado: tags=flaky
         """
         kernel_url = ('https://archives.fedoraproject.org/pub/archive/fedora'
                       '/linux/releases/29/Everything/x86_64/os/images/pxeboot'