Guarantee save-and-restore of GDBFLAGS on gdb.base/checkpoint-ns.exp

Guarantee save-and-restore of GDBFLAGS on gdb.base/checkpoint-ns.exp

Keith found out that several tests were failing when testing the
native-gdbserver board on Fedora (x86_64).  Strangely, these failures
had not been reported by our BuildBot.  Later, he found that the reason
for this was because the failures only happened when running the
testsuite without FORCE_PARALLEL (i.e., on serial mode; maybe it would
be worth having a builder testing things on serial...).  Then, he
decided to start bisecting the changes to see which one introduced the
failure (it was not trivial to know this only by looking at gdb.log).

After a lot of time, he found that Pedro's commit
e1316e60d4d1fe406efc6e7536b2bdb43733e9d2 was the culprit.  There was
nothing wrong in the code, but the new gdb.base/checkpoint-ns.exp
testcase did something that left the GDBFLAGS variable in an
inconsistent state.  This test works by modifying this variable to set
non-stop on, sourcing gdb.base/checkpoint.exp (which does the hard
work), and then restoring the old value on GDBFLAGS.  However, this was
not working because gdb.base/checkpoint.exp bails out if it is being
tested on gdbserver, and when it calls "continue" the control goes back
to the function calling the tests, and not to
gdb.base/checkpoint-ns.exp.

The fix is simple: just wrap the "source" call, and make
gdb.base/checkpoint-ns.exp aware of the "continue"/"return" calls made
by gdb.base/checkpoint.exp.

gdb/testsuite/ChangeLog:
2015-08-12  Sergio Durigan Junior  <sergiodj@redhat.com>
	    Pedro Alves  <palves@redhat.com>
	    Keith Seitz  <keiths@redhat.com>

	* gdb.base/checkpoint-ns.exp: Use save_vars to save and restore
	GDBFLAGS.