[gdb/testsuite] Relax breakpoint count check in gdb.python/py-rbreak.exp

[gdb/testsuite] Relax breakpoint count check in gdb.python/py-rbreak.exp

With a gdb 13.2 based package on SLE-15 aarch64,  I run into:
...
(gdb) PASS: gdb.python/py-rbreak.exp: nosharedlibrary
py sl = gdb.rbreak("^[^_]",minsyms=False)^M
Breakpoint 2 at 0x4004ac: file ../sysdeps/aarch64/crti.S, line 63.^M
  ...
(gdb) py print(len(sl))^M
12^M
(gdb) FAIL: gdb.python/py-rbreak.exp: check number of returned breakpoints is 11
...

The FAIL is due to:
- the glibc object crti.o containing debug information for function
  call_weak_fn, and
- the test-case not expecting this.

The debug information is there due to compiling glibc using a binutils which
contains commit 591cc9fbbfd ("gas/Dwarf: record functions").

I've run into a similar issue before, see commit 3fbbcf473a5 ("[gdb/testsuite]
Fix regexp in py-rbreak.exp").

The fix I applied there was to use a regexp "^[^_]" to filter out
__libc_csu_fini and __libc_csu_init, but that doesn't work for call_weak_fn.

Fix this by:
- reverting the regexp to "", and
- rewriting the check to require at least 11 functions, rather than a precise
  match.

Tested on x86_64-linux.

PR testsuite/30538
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30538