add some test cases for util package

[javeme]

Change-Id: I39b457c2a7a338a909b3aa0ba8ff4a8b5057b339

[javeme]

Before adding test cases:

[codecov[bot]]

Codecov Report

Merging #70 (9da2894) into master (265f74b) will increase coverage by 2.53%. The diff coverage is 90.90%.

@@             Coverage Diff              @@
##             master      #70      +/-   ##
============================================
+ Coverage     85.08%   87.61%   +2.53%     
- Complexity      793      829      +36     
============================================
  Files            61       61              
  Lines          2299     2326      +27     
  Branches        341      352      +11     
============================================
+ Hits           1956     2038      +82     
+ Misses          192      147      -45     
+ Partials        151      141      -10     

Impacted Files	Coverage Δ	Complexity Δ
...ain/java/com/baidu/hugegraph/util/NumericUtil.java	90.51% <80.00%> (+6.91%)	82.00 <13.00> (+14.00)
...ain/java/com/baidu/hugegraph/util/VersionUtil.java	83.72% <90.47%> (+24.26%)	10.00 <1.00> (+1.00)
.../java/com/baidu/hugegraph/util/CollectionUtil.java	98.11% <100.00%> (+13.57%)	42.00 <7.00> (+11.00)
...c/main/java/com/baidu/hugegraph/util/DateUtil.java	86.36% <100.00%> (+10.36%)	9.00 <0.00> (ø)
.../main/java/com/baidu/hugegraph/event/EventHub.java	81.94% <0.00%> (+1.38%)	27.00% <0.00%> (+1.00%)
...n/java/com/baidu/hugegraph/concurrent/KeyLock.java	80.39% <0.00%> (+1.96%)	26.00% <0.00%> (+1.00%)
.../main/java/com/baidu/hugegraph/perf/Stopwatch.java	87.75% <0.00%> (+2.04%)	21.00% <0.00%> (+1.00%)
...main/java/com/baidu/hugegraph/util/StringUtil.java	93.75% <0.00%> (+10.41%)	6.00% <0.00%> (ø%)
... and 4 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 265f74b...9da2894. Read the comment docs.

[javeme]

After adding test cases: