[gdb/testsuite] Fix stepi pattern in gdb.btrace/reconnect.exp

When running gdb.btrace/reconnect.exp, I run into the follow FAIL:
...
(gdb) stepi 19^M
0x00007ffff7dd8b67      59        return (ElfW(Addr)) &_DYNAMIC - \
  elf_machine_dynamic ();^M
(gdb) FAIL: gdb.btrace/reconnect.exp: first: stepi 19
...

The corresponding test looks like:
...
  gdb_test "stepi 19" "0x.* in .* from .*"
...
which matches the usual:
...
(gdb) stepi 19^M
0x00007ffff7dd8b67 in _dl_start () from target:/lib64/ld-linux-x86-64.so.2^M
(gdb) PASS: gdb.btrace/reconnect.exp: first: stepi 19
...
which I also get when removing configure flag
--with-separate-debug-dir=/usr/lib/debug.

Fix this by allowing the source line pattern in the test regexp.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-03-11  Tom de Vries  <tdevries@suse.de>

	* gdb.btrace/reconnect.exp: Allow source line pattern after stepi.