[gdb/testsuite] Add missing initial prompt read in multidictionary.exp

[gdb/testsuite] Add missing initial prompt read in multidictionary.exp

When running multidictionary.exp in conjunction with:
...
$ stress -c $(($(cat /proc/cpuinfo | grep -c "^processor") + 1))
...
we get:
...
Running gdb/testsuite/gdb.dwarf2/multidictionary.exp ...
ERROR: Couldn't load multidictionary into gdb.

                === gdb Summary ===

nr of unresolved testcases       1
...

The multidictionary test-case needs -readnow, and achieves this using:
...
gdb_spawn_with_cmdline_opts "-readnow"
gdb_load
...
but the initial gdb prompt is not read.  Usually, the following gdb_load
command accidentally consumes that initial prompt (at the gdb_expect for the
kill command in gdb_file_cmd).  But under high load, that doesn't happen and
we run into the error.

Fix this by consuming the initial gdb prompt after spawning gdb.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2019-07-23  Tom de Vries  <tdevries@suse.de>

	PR testsuite/24842
	* gdb.dwarf2/multidictionary.exp: Consume initial prompt after
	gdb_spawn_with_cmdline_opts.