[gdb/testsuite] Fix solib-list.exp test-case for exec with debug-info

[gdb/testsuite] Fix solib-list.exp test-case for exec with debug-info

Since commit a2fedca99c "Implement 'set/show exec-file-mismatch'.", I see the
following regression on openSUSE Leap 15.1:
...
FAIL: gdb.server/solib-list.exp: non-stop 0: target remote \
  (got interactive prompt)
FAIL: gdb.server/solib-list.exp: non-stop 1: target remote \
  (got interactive prompt)
...

The first FAIL in more detail:
...
(gdb) PASS: gdb.server/solib-list.exp: non-stop 0: file binfile
target remote localhost:2346
Remote debugging using localhost:2346
warning: Mismatch between current exec-file /data/gdb_versions/devel/build/\
  gdb/testsuite/outputs/gdb.server/solib-list/solib-list
and automatically determined exec-file /lib64/ld-2.26.so
exec-file-mismatch handling is currently "ask"
Load new symbol table from "/lib64/ld-2.26.so"? (y or n) n
warning: loading /lib64/ld-2.26.so Not confirmed.
Reading /lib64/ld-linux-x86-64.so.2 from remote target...
warning: File transfers from remote targets can be slow. \
  Use "set sysroot" to access files locally instead.
Reading /lib64/ld-linux-x86-64.so.2 from remote target...
Reading symbols from target:/lib64/ld-linux-x86-64.so.2...
Reading /lib64/ld-2.26.so-2.26-lp151.18.7.x86_64.debug from remote target...
Reading /lib64/.debug/ld-2.26.so-2.26-lp151.18.7.x86_64.debug from remote \
  target...
Reading /data/gdb_versions/devel/install/lib64/debug//lib64/\
  ld-2.26.so-2.26-lp151.18.7.x86_64.debug from remote target...
Reading /data/gdb_versions/devel/install/lib64/debug/lib64/\
  /ld-2.26.so-2.26-lp151.18.7.x86_64.debug from remote target...
Reading target:/data/gdb_versions/devel/install/lib64/debug/lib64/\
  /ld-2.26.so-2.26-lp151.18.7.x86_64.debug from remote target...
(No debugging symbols found in target:/lib64/ld-linux-x86-64.so.2)
0x00007ffff7dd7ea0 in ?? ()
(gdb) FAIL: gdb.server/solib-list.exp: non-stop 0: target remote (got \
  interactive prompt)
...

The commit introduces the "Load new symbol table from" question, and
gdb_test_multiple defaults to answering "no" and reporting the
"got interactive prompt" FAIL.

This FAIL is not seen on f.i. debian 10.2.  The difference originates from the
fact that the solib-list executable has debug-info in the openSUSE case, while
it doesn't in the debian case.

We can prevent the failure on openSUSE by stripping the executable from
debug-info:
...
+ exec strip --strip-debug ${binfile}
...

The difference in behaviour is a bug or improvement opportunity in the
exec-file-mismatch, filed as PR25475.

This patch fixes the FAIL by handling the question in the test-case.

Tested on x86_64-linux.

Tested on x86_64-linux with the gdbserver part of the patch introducing the
test-case reverted to ensure that this still FAILs.

gdb/testsuite/ChangeLog:

2020-03-15  Tom de Vries  <tdevries@suse.de>

	* gdb.server/solib-list.exp: Handle
	'Load new symbol table from "/lib64/ld-2.26.so"? (y or n)'.