snaps tests fail because of the new current link

[Leo Arias]

snapd in zesty now creates a symlink from ~/snap/<name>/current to ~/snap/<name>/<revision>. This is causing the busybox and mosquitto snaps tests fail:

======================================================================
ERROR: demos_tests.test_busybox.BusyBoxTestCase.test_busybox
----------------------------------------------------------------------
output: {{{b"rm: cannot remove '/home/ubuntu/snap/busybox/x1/busybox.test': No such file or directory\n"}}}

Traceback (most recent call last):
  File "/tmp/autopkgtest.AovD16/build.Nh9/snapcraft/snaps_tests/__init__.py", line 242, in run_command_in_snappy_testbed
    return self.snappy_testbed.run_command(command, cwd)
  File "/tmp/autopkgtest.AovD16/build.Nh9/snapcraft/snaps_tests/testbed.py", line 40, in run_command
    stderr=subprocess.STDOUT).decode('utf-8')
  File "/usr/lib/python3.5/subprocess.py", line 316, in check_output
    **kwargs).stdout
  File "/usr/lib/python3.5/subprocess.py", line 398, in run
    output=stdout, stderr=stderr)
subprocess.CalledProcessError: Command 'rm ~/snap/busybox/*/busybox.test' returned non-zero exit status 1

======================================================================
FAIL: demos_tests.test_mosquitto.MosquittoTestCase.test_mosquitto
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/tmp/autopkgtest.AovD16/build.Nh9/snapcraft/snaps_tests/demos_tests/test_mosquitto.py", line 46, in test_mosquitto
    'MQTT subscriber connected.\n'
  File "/tmp/autopkgtest.AovD16/build.Nh9/snapcraft/snaps_tests/__init__.py", line 231, in assert_command_in_snappy_testbed
    self.assertEqual(expected_output, output)
  File "/usr/lib/python3/dist-packages/testtools/testcase.py", line 350, in assertEqual
    self.assertThat(observed, matcher, message)
  File "/usr/lib/python3/dist-packages/testtools/testcase.py", line 435, in assertThat
    raise mismatch_error
testtools.matchers._impl.MismatchError: !=:
reference = '''\
MQTT subscriber connected.
test-mosquitto-topic b'test-message'
test-mosquitto-topic b'exit'
'''
actual = '''\
MQTT subscriber connected.
test-mosquitto-topic b'test-message'
test-mosquitto-topic b'exit'
MQTT subscriber connected.
test-mosquitto-topic b'test-message'
test-mosquitto-topic b'exit'
'''

[Leo Arias (elopio)]

Priority set to critical because this would block landings on the archive.

[Leo Arias (elopio)]

https://github.com/snapcore/snapcraft/pull/1187