[gdb/testsuite] Fix gdb.reverse/test_ioctl_TCSETSW.exp with libc debuginfo

[gdb/testsuite] Fix gdb.reverse/test_ioctl_TCSETSW.exp with libc debuginfo

When running test-case gdb.reverse/test_ioctl_TCSETSW.exp with glibc debuginfo
installed, I run into:
...
(gdb) PASS: gdb.reverse/test_ioctl_TCSETSW.exp: at TCSETSW call
step^M
__tcsetattr (fd=0, optional_actions=1, termios_p=0x7fffffffcf50) at \
  ../sysdeps/unix/sysv/linux/tcsetattr.c:45^M
45      {^M
(gdb) FAIL: gdb.reverse/test_ioctl_TCSETSW.exp: handle TCSETSW
...

The problem is that the step is expected to step over the call to tcsetattr,
but due to glibc debuginfo being installed, we step into the call.

Fix this by using next instead of step.

Tested on x86_64-linux.