With "maint set target-non-stop on" we get:
 -PASS: gdb.threads/signal-while-stepping-over-bp-other-thread.exp: step
 +FAIL: gdb.threads/signal-while-stepping-over-bp-other-thread.exp: step
The issue is simply that switch_back_to_stepped_thread is not used in
non-stop mode, thus infrun doesn't output the expected "switching back
to stepped thread" log.
gdb/testsuite/ChangeLog:
2015-08-07  Pedro Alves  <palves@redhat.com>
	* signal-while-stepping-over-bp-other-thread.exp: Expect "restart
	threads" as alternative to "switching back to stepped thread".
+2015-08-07  Pedro Alves  <palves@redhat.com>
+
+       * signal-while-stepping-over-bp-other-thread.exp: Expect "restart
+       threads" as alternative to "switching back to stepped thread".
+
 2015-08-07  Pedro Alves  <palves@redhat.com>
 
        * NEWS: Mention "maint set/show target-non-stop".
 
     "need to step-over"
     "resume \\(step=1"
     "signal arrived while stepping over breakpoint"
-    "switching back to stepped thread"
+    "(restart threads|switching back to stepped thread)"
     "stepped to a different line"
     "callme"
 }