[gdb/testsuite] Fix gdb.cp/step-and-next-inline.exp with -fcf-protection

[gdb/testsuite] Fix gdb.cp/step-and-next-inline.exp with -fcf-protection

On Ubuntu 22.04.1 x86_64, I run into:
...
(gdb) PASS: gdb.cp/step-and-next-inline.exp: no_header: not in inline 1
next^M
51        if (t != NULL^M
(gdb) FAIL: gdb.cp/step-and-next-inline.exp: no_header: next step 1
...

This is due to -fcf-protection, which adds the endbr64 at the start of get_alias_set:
...
0000000000001180 <_Z13get_alias_setP4tree>:
    1180:       f3 0f 1e fa             endbr64
    1184:       48 85 ff                test   %rdi,%rdi
...
so the extra insn gets an is-stmt line number entry:
...
INDEX  LINE   ADDRESS            IS-STMT PROLOGUE-END
  ...
11     50     0x0000000000001180 Y
12     50     0x0000000000001180
13     51     0x0000000000001184 Y
14     54     0x0000000000001184
...
and when stepping into get_alias_set we step to line 50:
...
(gdb) PASS: gdb.cp/step-and-next-inline.exp: no_header: in main
step^M
get_alias_set (t=t@entry=0x555555558018 <xx>) at step-and-next-inline.cc:50^M
50      {^M
...

In contrast, with -fcf-protection=none, we get:
...
0000000000001170 <_Z13get_alias_setP4tree>:
    1170:       48 85 ff                test   %rdi,%rdi
...
and:
...
INDEX  LINE   ADDRESS            IS-STMT PROLOGUE-END
  ...
11     50     0x0000000000001170 Y
12     51     0x0000000000001170 Y
13     54     0x0000000000001170
...
so when stepping into get_alias_set we step to line 51:
...
(gdb) PASS: gdb.cp/step-and-next-inline.exp: no_header: in main
step^M
get_alias_set (t=t@entry=0x555555558018 <xx>) at step-and-next-inline.cc:51^M
51        if (t != NULL^M
...

Fix this by rewriting the gdb_test issuing the step command to check which
line the step lands on, and issuing an extra next if needed.

Tested on x86_64-linux, both with and without -fcf-protection=none.

PR testsuite/29920
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29920