testsuite: Mark posix002 as fragile in threaded2 way

(cherry picked from commit ed2cf61f)

tests/libposix/all.T

-test('posix002', [ reqlib('unix'), omit_ways(prof_ways) ],
+test('posix002', [ reqlib('unix'), omit_ways(prof_ways), fragile_for(16550, ['threaded2']) ],
                  compile_and_run, [''])
 
 # Skip on mingw32: assumes existence of 'pwd' and /tmp