runtest: Treat test timeouts as fatal

If a test times out, that means it hasn't returned a prompt within
reasonable time.  Continuing to type at it is unlikely to be helpful,
since the lack of a prompt indicates that the subprocess has hung, so
subsequent tests are likely to time out as well.

This is particularly annoying in step 5, where the line that usually
times out, "(def! res2 (sum2 10000 0))" has its output ignored, so a
timeout is treated as success, meaning that runtest proceeds to the next
test, which of course also times out.

runtest.py

@@ -264,6 +264,9 @@ def assert_prompt(runner, prompts, timeout):
 soft_fail_cnt = 0
 failures = []
 
+class TestTimeout(Exception):
+    pass
+
 while t.next():
     if args.deferrable == False and t.deferrable:
         log(t.deferrable)
@@ -293,7 +296,10 @@ def assert_prompt(runner, prompts, timeout):
         res = r.read_to_prompt(['\r\n[^\s()<>]+> ', '\n[^\s()<>]+> '],
                                 timeout=args.test_timeout)
         #print "%s,%s,%s" % (idx, repr(p.before), repr(p.after))
-        if (t.ret == "" and t.out == ""):
+        if (res == None):
+            log(" -> TIMEOUT (line %d)" % t.line_num)
+            raise TestTimeout("TIMEOUT (line %d)" % t.line_num)
+        elif (t.ret == "" and t.out == ""):
             log(" -> SUCCESS (result ignored)")
             pass_cnt += 1
         elif (re.search(expects[0], res, re.S) or