[gdb/testsuite, 2/2] Fix gdb.linespec/explicit.exp with check-read1

[gdb/testsuite, 2/2] Fix gdb.linespec/explicit.exp with check-read1

When running gdb.linespec/explicit.exp with check-read1, we get:
...
(gdb) PASS: gdb.linespec/explicit.exp: set max-completions unlimited
break \a
-function
  ...
top
(gdb) PASS: gdb.linespec/explicit.exp: complete with no arguments
break
-function
 ...
top
(gdb) FAIL: gdb.linespec/explicit.exp: complete with no arguments (clearing input line)
...

The problem is that the send_gdb "\t\t" triggers completion twice:
...
        set tst "complete with no arguments"
        send_gdb "break \t"
        gdb_test_multiple "" $tst {
            "break \\\x07" {
                send_gdb "\t\t"
                gdb_test_multiple "" $tst {
	...
	}
	clear_input_line $tst
...
but the following gdb_test_multiple only parses it once, so the second
completion is left for clear_input_line, which fails.

Fix this by triggering completion only once.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2019-07-29  Tom de Vries  <tdevries@suse.de>

	* gdb.linespec/explicit.exp: Fix completion trigger for "complete with
	no arguments".