Fix hanging of popen.test

The "open-output-pipe":"no duplicate" test has been hanging, on and
off, and not completely reliably, for a few years.  It's now doing so
fairly reliably for me, and investigation shows that

- the child shell process is in a tight loop (99% CPU)

- the parent Guile process is stuck calling waitpid().

The problem is that the child hasn't got the SIGPIPE that the test
intends, and so is continuing to echo "closed" forever; and Guile is
waiting for it to terminate, forever.

I haven't fully debugged the SIGPIPE problem, but it sounds very like
what Chet Ramey describes here:
http://old.nabble.com/Re%3A-SIGPIPE-not-properly-reset-with-%27trap---PIPE%27-p20985595.html.

(And my version of bash is 3.2.39.)

So, a fix should be to use something other than shell to implement the
child; and it appears that this works.

* check-guile.in (TEST_SUITE_DIR): Export.

* test-suite/tests/popen-child.scm: New script file.

* test-suite/tests/popen.test ("open-output-pipe", "no duplicate"):
  Use Guile for the child process, instead of shell.

check-guile.in

@@ -15,6 +15,7 @@ top_builddir=@top_builddir_absolute@
 top_srcdir=@top_srcdir_absolute@
 
 TEST_SUITE_DIR=${top_srcdir}/test-suite
+export TEST_SUITE_DIR
 
 if [ x"$1" = x-i ] ; then
     guile=$2