oeqa/selftest: Don't use cleanup-workdir

oeqa/selftest: Don't use cleanup-workdir

cleanup-workdir isn't a partcularly good way to attempt to cleanup after
tests and in some cases is actively breaking the workdir. Whilst this is
a bug in cleanup-workdir, I'd prefer it didn't break tests, particularly
when as far as I can see, these calls are "belt and braces" and don't appear
to serve a specific purpose. If this introduces bugs, we should fix this
bugs.

For the purposes of history, binutils-cross-i586 gets run do_fetch to
do_populate_sysroot by:

$ oe-selftest -r devtool.DevtoolTests.test_devtool_virtual_kernel_modify

then:

$ oe-selftest -r oescripts.BuildhistoryDiffTests.test_buildhistory_diff

wipes out the contents of tmp/work/x86_64-linux/binutils-cross-i586/2.28-r0/
but does not wipe out the corresponding stamps, then:

$ oe-selftest -r runtime-test.Postinst.test_postinst_rootfs_and_boot

needs binutils-cross-i586:do_populate_lic and if and only if this is
unavailable from sstate, it fails since it thinks the source is already
unpacked when it isn't resulting in:

WARNING: binutils-cross-i586-2.28-r0 do_populate_lic: Could not copy license file /home/pokybuild/yocto-autobuilder/yocto-worker/nightly-oe-selftest/build/build/tmp/work/x86_64-linux/binutils-cross-i586/2.28-r0/git/COPYING3.LIB to /home/pokybuild/yocto-autobuilder/yocto-worker/nightly-oe-selftest/build/build/tmp/work/x86_64-linux/binutils-cross-i586/2.28-r0/license-destdir/binutils-cross-i586/COPYING3.LIB: [Errno 2] No such file or directory: '/home/pokybuild/yocto-autobuilder/yocto-worker/nightly-oe-selftest/build/build/tmp/work/x86_64-linux/binutils-cross-i586/2.28-r0/git/COPYING3.LIB'
ERROR: binutils-cross-i586-2.28-r0 do_populate_lic: QA Issue: binutils-cross-i586: LIC_FILES_CHKSUM points to an invalid file: /home/pokybuild/yocto-autobuilder/yocto-worker/nightly-oe-selftest/build/build/tmp/work/x86_64-linux/binutils-cross-i586/2.28-r0/git/COPYING3 [license-checksum]

and similar errors.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>