test/automake: don't use paralell test harness (since automake 1.12)

Starting from automake 1.2 there is parallel test harness, that redirects all
output to some log, which serial-test doesn't do.
So in case of new runner we can get no output for 10 minutes, for example on my
desktop:
  $ time make verify VERBOSE=1
  PASS: test/test-script.sh
  ============================================================================
  Testsuite summary for libevent 2.1.5-beta
  ============================================================================
  # TOTAL: 1
  # PASS:  1
  # SKIP:  0
  # XFAIL: 0
  # FAIL:  0
  # XPASS: 0
  # ERROR: 0
  ============================================================================

  real    25m31.735s
  user    0m13.753s
  sys     0m7.648s

And this means that this will fail on travis-ci, since it has timeout for 10
minutes. Sure we can use `travis wait 60` instead, but I think that it is
better to fix this by writing result to output, instead of hacking around, so
let's use serial-tests instead of parallel always.
And now it works on travis-ci under linux because it has automake 1.11 while
osx has at least 1.12.

Links:
https://docs.travis-ci.com/user/common-build-problems/
https://www.gnu.org/software/automake/manual/html_node/Serial-Test-Harness.html#Serial-Test-Harness
https://www.gnu.org/software/automake/manual/html_node/Parallel-Test-Harness.html

CI:
https://travis-ci.org/azat/libevent/jobs/129171497 # ok on linux
https://travis-ci.org/azat/libevent/jobs/129171532 # no output for 10 min on osx

Makefile.am

@@ -6,7 +6,7 @@
 
 # 'foreign' means that we're not enforcing GNU package rules strictly.
 # '1.9' means that we need automake 1.9 or later (and we do).
-AUTOMAKE_OPTIONS = foreign 1.9 subdir-objects
+AUTOMAKE_OPTIONS = foreign 1.9 subdir-objects serial-tests
 
 ACLOCAL_AMFLAGS = -I m4