test-only script caused spurious failure

[Dan Kegel]

Saw this just now:

+ echo run 2 of 5
+ do_subset_tests dlls/user32
+ subset_errors=0
run 2 of 5
+ pwd
+ export WINEPREFIX=/home/austin/wineslave.dir/sandbox/slave/runtests-heaptest/build/wineprefix-subset
+ rm -rf /home/austin/wineslave.dir/sandbox/slave/runtests-heaptest/build/wineprefix-subset
rm: cannot remove
`/home/austin/wineslave.dir/sandbox/slave/runtests-heaptest/build/wineprefix-subset':
Directory not empty
+ test -d dlls/user32/tests
+ cd dlls/user32/tests
+ make_highlit_log
+ make -k test
+ mstatus=2

Some process is alive, and keeping the directory from being deleted.

Probably need to kill wineserver at the bottom of the loop; had to do
that in do_foreground_tests.
Checked in a try at this as r2611, will test tonight.