sanity: Use random filename for maximum path length test

sanity: Use random filename for maximum path length test

check_create_long_filename used a fixed filename for its test files. This
meant that os.remove(testfile) could fail with ENOENT if two instances were
running at the same time against the same sstate directory. Using a
randomly generated filename stops this from happening.

(Although it might seem unlikely, this race did appear to occur multiple
times with Jenkins - presumably because the matrix jobs were all kicked off
at the same time.)

(From OE-Core rev: bc28e3f26e7f85af82f403924c0ae29e1ad34a87)

Signed-off-by: Mike Crowe <mac@mcrowe.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>