[gdb/testsuite] Fix mi-sym-info.exp with check-read1

When running gdb.mi/mi-sym-info.exp with check-read1, we run into:
...
FAIL: gdb.mi/mi-sym-info.exp: List all functions
FAIL: gdb.mi/mi-sym-info.exp: List all variables
...

The problem is that while the $mi_gdb_prompt is active, gdb_test_multiple is
used without -prompt "$mi_gdb_prompt$", so it defaults to matching $gdb_prompt.

Fix this by adding the missing gdb_test_multiple arguments.

Reg-tested on x86_64-linux with make targets check and check-read1.

gdb/testsuite/ChangeLog:

2020-03-03  Tom de Vries  <tdevries@suse.de>

	* gdb.mi/mi-sym-info.exp: Add missing -prompt "$mi_gdb_prompt$" to
	gdb_test_multiple calls.