[gdb/testsuite] Fix fortran module tests with stressed cpu

When running these test-cases:
- gdb.fortran/info-modules.exp
- gdb.fortran/module.exp
- gdb.mi/mi-fortran-modules.exp
in conjunction with:
...
$ stress -c $(($(cat /proc/cpuinfo | grep -c "^processor") + 1))
...
I run into timeouts.

Fix this by using:
- "set auto-solib-add off" to avoid symbols of shared libs
  (which doesn't work for libc, now that libpthread_name_p has been
  updated to  match libc)
- "nosharedlibrary" to avoid symbols of libc

Tested on x86_64-linux.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28133