[openacc, testsuite] Fix undefined behaviour in atomic_capture-1.c

2018-04-29  Julian Brown  <julian@codesourcery.com>
	    Tom de Vries  <tom@codesourcery.com>

	PR testsuite/85527
	* testsuite/libgomp.oacc-c-c++-common/atomic_capture-1.c: Allow
	arbitrary order for iterations of atomic subtract check.

Co-Authored-By: Tom de Vries <tom@codesourcery.com>

From-SVN: r259748