libgomp/testsuite: Improve omp_get_device_num() tests

libgomp/testsuite: Improve omp_get_device_num() tests

Related to r12-6208-gebc853deb7cc0487de9ef6e891a007ba853d1933
"libgomp: Fix GOMP_DEVICE_NUM_VAR stringification during offload image load"

That commit fixed an issue with omp_get_device_num() on gcn/nvptx that
resulted in having always the value 0.
This commit modifies the tests to iterate over all devices such that on a
multi-nonhost-device system it had detected that always-zero issue.

libgomp/ChangeLog:

	* testsuite/libgomp.c-c++-common/target-45.c: Iterate over all devices.
	* testsuite/libgomp.fortran/target10.f90: Likewise.