tests: fix races wrt output files of background processes

tests: fix races wrt output files of background processes

At least the false positive in tail-2/follow-stdin.sh could be seen
on a 4-core i5 system with -j8.  Fix similar cases. too.

* tests/tail-2/follow-stdin.sh: Empty the 'out' file in each iteration.
Otherwise, under heavy system load, 'check_tail_output' would see the
expected output from the previous round before tail would have the
chance to come up in the background.
While at it, move the creation of the 'exp' file out of the loop.
* tests/dd/stats.sh: Empty the output file of the background process
here, too.
* tests/misc/cat-buf.sh: Likewise.
* tests/misc/stdbuf.sh: Likewise.
* tests/tail-2/wait.sh: Likewise.