gdb/testsuite: fix FAIL in gdb.base/clear_non_user_bp.exp

Tom and Simon feedback that there is a test failing in this commit:

  commit a5c69b1e
  Date:   Sun Apr 17 15:09:46 2022 +0800

    gdb: fix using clear command to delete non-user breakpoints(PR cli/7161)

Then, I reproduced the same fail with Ubuntu 20.04 as Simon said, and I
fixed the nit in this patch.  The root of the problem is not correctly
matching the presentation of internal breakpoints.

In addition, as Pedro pointed out, the original testcase is not portable
in some methods, so this patch fixes this issue and some other
improvements.

Tested on x86_64 ubuntu 20.04.4 and openSUSE Tumbleweed(VERSION_ID="20220425").