Tighten regexp of lib/completion-support.exp:test_gdb_complete_tab_multiple

While writing the tests included in the previous commit, I noticed
that test_gdb_complete_tab_multiple would not FAIL if GDB happens to
show more completions than expected before the expected list.

E.g., with something like this, expecting "p foo" to complete to
"foo2" and "foo3":

 test_gdb_complete_tab_multiple "p foo" "" {
	"foo2"
	"foo3"
 }

and then if foo actually completes to:

 (gdb) p foo[TAB]
 foo1   foo2  foo3
 ^^^^

we'd still PASS.  (Note the spurious "foo1" above.)

This tightens the regexp with a beginning anchor thus making the
completions above cause a FAIL.  Other similar functions in
completion-support.exp already do something like this; I had just
missed this one originally.  Thankfully, this did not expose any
problems in the gdb.linespec/ tests.  Phew.

gdb/testsuite/ChangeLog:
2017-12-13  Pedro Alves  <palves@redhat.com>

	* lib/completion-support.exp (test_gdb_complete_tab_multiple):
	Tighten regexp by matching with an anchor.