oeqa/selftest: Add test for conflicting sysroot provider

sysroot-test depends on virtual/sysroot-test which we build for one machine,
switch machine, switch provider of virtual/sysroot-test and check that the
sysroot is correctly cleaned up. The files in the two providers overlap
so can cause errors if the sysroot code doesn't function correctly.

Yes, sysroot-test should be machine specific really to avoid this, however
the sysroot cleanup should also work.

This adds a test for bug:

[YOCTO #13702]

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