Fix racy output matching in gdb.base/cpcompletion.exp

Fix racy output matching in gdb.base/cpcompletion.exp

With:

 $ make check-read1 TESTS="gdb.cp/cpcompletion.exp"

we get (from gdb.log):

 (gdb) complete break Foo::
 break Foo::Foo()
 break Foo::Foofoo()
 break Foo::get_foo()
 break Foo::set_foo(int)
 break Foo::~Foo()
 (gdb) FAIL: gdb.cp/cpcompletion.exp: complete class methods (Foo not found)

The problem is that the
  "break ${class}::\[A-Za-z0-9_~\]+"
regexp patches partial input, like:
  break Foo::F
  break Foo::Fo
  break Foo::Foo
etc.

Fix that by expecting each whole line.

gdb/testsuite/ChangeLog:
2017-11-09  Pedro Alves  <palves@redhat.com>

	* gdb.cp/cpcompletion.exp (test_class_complete): Tighten regex to
	match till end of line.