gdb/testsuite: make some tests in gdb.base enable non-stop using GDBFLAGS

For the same reason as explained in commit 7cb2893d ("gdb/testsuite:
gdb.mi/mi-nonstop-exit.exp: enable non-stop using GDBFLAGS").

Note that the use of

    set GDBFLAGS "$GDBFLAGS ..."

instead of

    append GDBFLAGS "..."

is intentional.  "append" is silent when appending to a non-existent
variable.  So if this code if moved to a proc (as is the case already
for step-sw-breakpoint-adjust-pc.exp) and we forget to add "global
GDBFLAGS", the flag won't be added to the global GDBFLAGS, and we won't
actually enable non-stop, and it might go unnoticed.  Using the "set"
version will turn into an error if we forget the "global".

This makes these test work correctly with native-extended-gdbserver.
Some of them were silently failing because we runto_main is silent when
it fails.

gdb/testsuite/ChangeLog:

	* gdb.base/async-shell.exp: Enable non-stop through GDBFLAGS.
	* gdb.base/continue-all-already-running.exp: Likewise.
	* gdb.base/moribund-step.exp: Likewise.
	* gdb.base/step-sw-breakpoint-adjust-pc.exp: Likewise.

Change-Id: I19ef05d07a0ec4a9c9476af2ba6e1ea1159ee437