[gdb/testsuite] Use early-out style in gdb.base/break-probes.exp

[gdb/testsuite] Use early-out style in gdb.base/break-probes.exp

Reduce indentation and improve readability in test-case
gdb.base/break-probes.exp by replacing:
...
if { <cond> } {
  <lots-of-code>
}
...
with:
...
if { ! <cond> } {
  return -1
}
<lots-of-code>
...

Tested on x86_64-linux.