[gdb/testsuite] Fix gdb.threads/dlopen-libpthread.exp for upstream glibc, again

[gdb/testsuite] Fix gdb.threads/dlopen-libpthread.exp for upstream glibc, again

On an x86_64 laptop running ubuntu 22.04.1 with unity desktop:
...
$ echo $XDG_CURRENT_DESKTOP
Unity:Unity7:ubuntu
...
I have:
...
$ echo $LD_PRELOAD
libgtk3-nocsd.so.0
...
due to package gtk3-nocsd, a package recommended by unity-session.

Consequently, for each exec these dependencies are pulled in, including
libpthread.so.0:
...
$ lddtree /lib/x86_64-linux-gnu/libgtk3-nocsd.so.0
libgtk3-nocsd.so.0 => /lib/x86_64-linux-gnu/libgtk3-nocsd.so.0 (interpreter => none)
    libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2
    libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0
    libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6
        ld-linux-x86-64.so.2 => /lib/x86_64-linux-gnu/ld-linux-x86-64.so.2
...

So, while test-case gdb.threads/dlopen-libpthread.exp appears to run ok:
...
 # of expected passes		12
 # of unsupported tests		1
...
with LD_PRELOAD="" we have instead:
...
(gdb) PASS: gdb.threads/dlopen-libpthread.exp: continue to breakpoint: notify
info sharedlibrary^M
From  To                  Syms Read   Shared Object Library^M
$hex  $hex  Yes         /lib64/ld-linux-x86-64.so.2^M
$hex  $hex  Yes         /lib/x86_64-linux-gnu/libc.so.6^M
$hex  $hex  Yes         dlopen-libpthread.so^M
(gdb) FAIL: gdb.threads/dlopen-libpthread.exp: libpthread.so found
...

The problem is that libpthread is expected as dependency of
dlopen-libpthread.so, but it's missing:
...
$ lddtree dlopen-libpthread.so
dlopen-libpthread.so => ./dlopen-libpthread.so (interpreter => none)
    libc.so.6 => $outputs/gdb.threads/dlopen-libpthread/dlopen-libpthread.so.d/libc.so.6
        ld-linux-x86-64.so.2 => /lib/x86_64-linux-gnu/ld-linux-x86-64.so.2
...
due to having glibc 2.35, which has libpthread integrated into libc.

Fix this by:
- adding a proc has_dependency
- using [has_dependency $exec libpthread.so] as hint that libpthread
  may be preloaded
- using ![has_dependency $shlib libpthread.so] to detect that
  the libpthread.so dependency is missing.

Also add a missing return after untested "no matching probes".

Tested on x86_64-linux, with and without LD_PRELOAD="".