[gdb/testsuite] Fix gdb.threads/access-mem-running-thread-exit.exp

In OBS (Open Build Service), with a 15.2 based gdb package, occasionally I run
into:
...
(gdb) inferior 2
[Switching to inferior 2 [process 31372] (access-mem-running-thread-exit)]
[Switching to thread 2.1 (Thread 0xf7db9700 (LWP 31372))](running)
(gdb) print global_var = 555
$1 = 555
(gdb) print global_var
$2 = 556
(gdb) FAIL: $exp: all-stop: access mem \
  (print global_var after writing, inf=2, iter=1)
...

I managed to reproduce this on current trunk using a reproducer patch (posted
in the PR).

The problem is due to commit 31c21e2c13d ("[gdb/testsuite] Fix
gdb.threads/access-mem-running-thread-exit.exp with clang"), which introduced
an increment of global_var at the start of main.

This created a race between:
- gdb modifying global_var, and
- the inferior modifying global_var.

Fix this by:
- adding a new empty function setup_done,
- adding a call to setup_done after the increment of global_var, and
- rather than running to main, running to setup_done.

Tested on x86_64-linux.

PR testsuite/32822
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32822

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