Testsuite: fully migrate to use_gdb_stub convenience func

Testsuite: fully migrate to use_gdb_stub convenience func

In the GDB test suite, there are still multiple invocations of
"target_info exists use_gdb_stub".  However, the recommended way of
checking for use_gdb_stub is to call the convenience function of the same
name.

Replace these occurrences and just call "use_gdb_stub" instead.

gdb/testsuite/ChangeLog:

	* gdb.ada/exec_changed.exp: Replace "target_info exists
	use_gdb_stub" by "use_gdb_stub".
	* gdb.ada/start.exp: Likewise.
	* gdb.base/async-shell.exp: Likewise.
	* gdb.base/attach-pie-misread.exp: Likewise.
	* gdb.base/attach-wait-input.exp: Likewise.
	* gdb.base/break-entry.exp: Likewise.
	* gdb.base/break-interp.exp: Likewise.
	* gdb.base/dprintf-detach.exp: Likewise.
	* gdb.base/nostdlib.exp: Likewise.
	* gdb.base/solib-nodir.exp: Likewise.
	* gdb.base/statistics.exp: Likewise.
	* gdb.base/testenv.exp: Likewise.
	* gdb.mi/mi-exec-run.exp: Likewise.
	* gdb.mi/mi-start.exp: Likewise.
	* gdb.multi/dummy-frame-restore.exp: Likewise.
	* gdb.multi/multi-arch-exec.exp: Likewise.
	* gdb.multi/multi-arch.exp: Likewise.
	* gdb.multi/tids.exp: Likewise.
	* gdb.multi/watchpoint-multi.exp: Likewise.
	* gdb.python/py-events.exp: Likewise.
	* gdb.threads/attach-into-signal.exp: Likewise.
	* gdb.threads/attach-stopped.exp: Likewise.
	* gdb.threads/threadapply.exp: Likewise.
	* lib/selftest-support.exp: Likewise.