[libgomp, openacc, testsuite] Fix async/wait logic in lib-13.f90

[libgomp, openacc, testsuite] Fix async/wait logic in lib-13.f90

The purpose of the lib-13.f90 test-case is to test acc_wait_all_async.  The
test indeed calls acc_wait_all_async, but then subsequentlys calls
acc_wait_all, so the acc_wait_all_async functionality is not tested.
Furthermore, all acc_async_test calls are placed in a location where they are
not guaranteed to succeed, which explains why there's an xfail for the lower
optimization levels.

This patch fixes the problems by replacing acc_wait_all with an acc_wait on
the async id used for the acc_wait_all_async call, and moving the
acc_async_test calls to the correct locations.

Reg-tested on x86_64 with nvptx accelerator.

2018-07-26  Tom de Vries  <tdevries@suse.de>

	* testsuite/libgomp.oacc-fortran/lib-13.f90: Replace acc_wait_all with
	acc_wait.  Move acc_async_test calls to correct locations.  Remove
	xfail.

From-SVN: r262989