[gdb/testsuite] Fix gdb.multi/attach-no-multi-process.exp with check-readmore

[gdb/testsuite] Fix gdb.multi/attach-no-multi-process.exp with check-readmore

When running test-case gdb.multi/attach-no-multi-process.exp with
check-readmore, I get:
...
(gdb) attach 13411^M
Attaching to Remote target^M
No unwaited-for children left.^M
(gdb) Reading symbols from attach-no-multi-process...^M
Reading symbols from /lib64/libm.so.6...^M
(No debugging symbols found in /lib64/libm.so.6)^M
Reading symbols from /lib64/libc.so.6...^M
(No debugging symbols found in /lib64/libc.so.6)^M
Reading symbols from /lib64/ld-linux-x86-64.so.2...^M
(No debugging symbols found in /lib64/ld-linux-x86-64.so.2)^M
0x00007f5df1fffc8a in clock_nanosleep@GLIBC_2.2.5 () from /lib64/libc.so.6^M
FAIL: gdb.multi/attach-no-multi-process.exp: target_non_stop=off: \
  attach to the program via remote (timeout)
...

The problem is that the attach output is matched using gdb_test, which uses
the '$gdb_prompt $' regexp, and this does not handle the case that '(gdb) ' is
not the last available output.

Fix this by using a gdb_test_multiple instead with a '$gdb_prompt ' regexp, so
without the '$' anchor.

Tested on x86_64-linux with native, check-read1 and check-readmore.