[gdb/testsuite] Fix gdb.dwarf2/frame-inlined-in-outer-frame.exp

[gdb/testsuite] Fix gdb.dwarf2/frame-inlined-in-outer-frame.exp

I'm running into the following FAIL:
...
(gdb) starti ^M
Starting program: frame-inlined-in-outer-frame frame^M
^M
^M
Program stopped.^M
0x0000000000401000 in _start ()^M
(gdb) PASS: gdb.dwarf2/frame-inlined-in-outer-frame.exp: frame
frame^M
(gdb) FAIL: gdb.dwarf2/frame-inlined-in-outer-frame.exp: step into foo
stepi^M
0x0000000000401001 in foo ()^M
...

The problem is that the .exp file issues a gdb_starti_cmd without consuming
the resulting prompt.  Consequently, the gdb_test issuing the frame command
consumes that prompt, and things are out-of-sync from that point onwards.

Fix this by consuming the gdb prompt after gdb_starti_cmd.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-09-08  Tom de Vries  <tdevries@suse.de>

	* gdb.dwarf2/frame-inlined-in-outer-frame.exp: Consume gdb prompt
	after gdb_starti_cmd.