Flaky test TraceAnalyzerTest

I get multiple failures in TraceAnalyzerTest every time I run `make check` locally.

Example failure:

```
$ cat t/log-run-trace_analyzer_test-TraceAnalyzerTest.Merge
Note: Google Test filter = TraceAnalyzerTest.Merge
[==========] Running 1 test from 1 test case.
[----------] Global test environment set-up.
[----------] 1 test from TraceAnalyzerTest
[ RUN ] TraceAnalyzerTest.Merge
tools/trace_analyzer_test.cc:165: Failure
Expected equality of these values:
cnt.size()
Which is: 1
result.size()
Which is: 2
terminate called after throwing an instance of 'testing::internal::GoogleTestFailureException'
what(): tools/trace_analyzer_test.cc:165: Failure
Expected equality of these values:
cnt.size()
Which is: 1
result.size()
Which is: 2
t/run-trace_analyzer_test-TraceAnalyzerTest.Merge: line 4: 2794137 Aborted (core dumped) TEST_TMPDIR=$d ./trace_analyzer_test --gtest_filter=TraceAnalyzerTest.Merge
$ cat /dev/shm/rocksdb.Dn29t/run-trace_analyzer_test-TraceAnalyzerTest.Merge/trace_analyzer_test_2794137_289213897083967911/merge/test-qps_stats.txt
0 0 0 0 0 1 0 0 0 1
0 0 0 0 0 0 0 0 0 0
$
```

Probably due to this:

```
uint32_t duration =
static_cast((end_time_ - begin_time_) / 1000000);
...
if (qps_f_) {
for (uint32_t i = 0; i < duration; i++) {
... (print a line to the file)
```