[gdb/testsuite] Silence warnings about obsolete -gstabs

When running test-case gdb.base/gdbindex-stabs.exp on openSUSE Tumbleweed (with
gcc 12) I get:
...
gdb compile failed, gdb/testsuite/gdb.base/gdbindex-stabs.c: warning: \
  STABS debugging information is obsolete and not supported anymore
...

Silence the warning by passing quiet to gdb_compile.  Likewise in two other
test-cases.