selftest/cases/devtool.py: fix workspace layer checking

selftest/cases/devtool.py: fix workspace layer checking

Fixed:
$ oe-selftest -r devtool.DevtoolTests.test_create_workspace
[snip]
2017-12-20 18:28:59,404 - oe-selftest - INFO - Traceback (most recent call last):
  File "/workspace2/lyang1/poky/meta/lib/oeqa/core/decorator/__init__.py", line 32, in wrapped_f
    return func(*args, **kwargs)
  File "/workspace2/lyang1/poky/meta/lib/oeqa/selftest/cases/devtool.py", line 177, in test_create_workspace
    self.assertTrue('/workspace' not in result.output, 'This test cannot be run with a workspace layer in bblayers.conf')
AssertionError: False is not true : This test cannot be run with a workspace layer in bblayers.conf
[snip]

$ bitbake-layers show-layers
NOTE: Starting bitbake server...
layer                 path                                      priority
==========================================================================
meta                  /workspace2/lyang1/poky/meta              5
meta-poky             /workspace2/lyang1/poky/meta-poky         5
meta-yocto-bsp        /workspace2/lyang1/poky/meta-yocto-bsp    5
meta-selftest         /workspace2/lyang1/poky/meta-selftest     5

There is no workspace layer, but I'm in /workspace2, this patch can fix the problem.

[YOCTO #12442]

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>