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

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

With test-case gdb.mi/mi-disassemble.exp and check-read1 I run into:
...
FAIL: gdb.mi/mi-disassemble.exp: disassemble /b main
FAIL: gdb.mi/mi-disassemble.exp: get valueof "*((unsigned char *) 0x400549)"
...

The problem for both FAILs is that the output is parsed using
gdb_test_multiple, which has implicit clauses using $gdb_prompt, which can
match before the explicit clauses using $mi_gdb_prompt.

Fix this by passing -prompt "$mi_gdb_prompt$" to gdb_test_multiple.

Tested on x86-64-linux.