[gdb/testsuite] Expect conformation question in gdb.server/solib-list.exp

Before commit a8654e7d 'Fixes PR 25475: ensure exec-file-mismatch "ask"
always asks in case of mismatch', there was a difference in behaviour in
test-case gdb.server/solib-list.exp.

If the executable did not contain debug info (as is usually the case), gdb
would detect a mismatch but not ask for confirmation:
...
(gdb) target remote localhost:2346^M
Remote debugging using localhost:2346^M
warning: Mismatch between current exec-file solib-list^M
and automatically determined exec-file /lib64/ld-2.26.so^M
exec-file-mismatch handling is currently "ask"^M
Reading symbols from /lib64/ld-2.26.so...^M
Reading symbols from /usr/lib/debug/lib64/ld-2.26.so.debug...^M
0x00007ffff7dd7ea0 in _start () at rtld.c:745^M
745     }^M
(gdb) PASS: gdb.server/solib-list.exp: non-stop 0: target remote
...

If the executable did contain debug info (as happens to be the case for
openSUSE), gdb would detect a mismatch and ask for confirmation:
...
(gdb) PASS: gdb.server/solib-list.exp: non-stop 0: file binfile
target remote localhost:2346^M
Remote debugging using localhost:2346^M
warning: Mismatch between current exec-file solib-list^M
and automatically determined exec-file /lib64/ld-2.26.so^M
exec-file-mismatch handling is currently "ask"^M
Load new symbol table from "/lib64/ld-2.26.so"? (y or n) y^M
Reading symbols from /lib64/ld-2.26.so...^M
Reading symbols from /usr/lib/debug/lib64/ld-2.26.so.debug...^M
0x00007ffff7dd7ea0 in _start () at rtld.c:745^M
745     }^M
(gdb) PASS: gdb.server/solib-list.exp: non-stop 0: target remote
...

After commit a8654e7d, the confirmation is now also asked in case there's
no debug info.

Tighten the test-case by verifying that the confirmation question is asked, as
suggested in the log message of commit a8654e7d:
...
we can remove the bypass introduced by Tom in 6b9374f1, in order to always
answer to the 'load' question.
...

gdb/testsuite/ChangeLog:

2020-06-29  Tom de Vries  <tdevries@suse.de>

	PR gdb/25475
	* gdb.server/solib-list.exp: Verify that the symbol reload
	confirmation question is asked.