oeqa/selftest/devtool: abort if a local workspace already exist

oeqa/selftest/devtool: abort if a local workspace already exist

if user run devtool selftests with a local workspacelayer
the tests fail with various error such as:

- devtool.DevtoolAddTests.test_devtool_add just hangs
- devtool.DevtoolModifyTests.* fail with the following error:

 ERROR: Found duplicated BBFILE_COLLECTIONS 'workspacelayer', check bblayers.conf or layer.conf to fix it.
 Found duplicated BBFILE_COLLECTIONS 'workspacelayer', check bblayers.conf or layer.conf to fix it.

Check if a workspacelayer exists, warn the user and abort the tests

Signed-off-by: Julien Stephan <jstephan@baylibre.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>