[test] reduce the noise in task logs

See errors like:
https://ci.chromium.org/p/v8/builders/luci.v8.ci/V8%20Fuzzer/28117

The logs only shows the tail of the stderr, in order to get information, huge logs
need to be downloaded.

Instead of using the tail end of the stderr logs, we get the head of the stderr logs,
which is the important part for us.

R=machenbach@chromium.org
CC=​​​​​yangguo@chromium.org,sergiyb@chromium.org

gclient sync
cd v8
tools/jsfunfuzz/fuzz-harness.sh out/x64.debug/d8 fuzz-results.tar.bz2
```

Test: ```
Bug: v8:8493
Change-Id: Ia87c52f5b08ff9748cf2a81c9ca983d22fda650d
Reviewed-on: https://chromium-review.googlesource.com/c/1378177
Commit-Queue: Tamer Tas <tmrts@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58302}

tools/jsfunfuzz/fuzz-harness.sh

@@ -51,8 +51,17 @@ if [ "$3" == "--download" ]; then
     cat << EOF | patch -s -p0 -d "$v8_root"
 --- tools/jsfunfuzz/jsfunfuzz/multi_timed_run.py~
 +++ tools/jsfunfuzz/jsfunfuzz/multi_timed_run.py
-@@ -125,7 +125,7 @@
- 
+@@ -118,19 +118,19 @@
+-def showtail(logfilename):
++def showtail(logfilename, method="tail"):
+-   cmd = "tail -n 20 %s" % logfilename
++   cmd = "%s -n 20 %s" % (method, logfilename)
+    print cmd
+    print ""
+    os.system(cmd)
+    print ""
+    print ""
+
  def many_timed_runs():
      iteration = 0
 -    while True:
@@ -60,6 +69,12 @@ if [ "$3" == "--download" ]; then
          iteration += 1
          logfilename = "w%d" % iteration
          one_timed_run(logfilename)
+         if not succeeded(logfilename):
+             showtail(logfilename)
+-            showtail("err-" + logfilename)
++            showtail("err-" + logfilename, method="head")
+
+             many_timed_runs()
 EOF
   fi