etcdserver: api/v2v3: add initial tests

[philips]

Add a bunch of tests on fundamental operation of Create/Set/Delete.

Context: I want to use this package to run the discovery service against
the v3 storage backend. The limited functionality already implemented
should be sufficient to do this.
coreos/discovery.etcd.io#52

[codecov-io]

Codecov Report

Merging #10622 into master will increase coverage by 0.35%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #10622      +/-   ##
==========================================
+ Coverage   71.62%   71.98%   +0.35%     
==========================================
  Files         393      393              
  Lines       36571    36628      +57     
==========================================
+ Hits        26194    26365     +171     
+ Misses       8538     8405     -133     
- Partials     1839     1858      +19

Impacted Files	Coverage Δ
proxy/grpcproxy/metrics.go	25% <0%> (-75%)	⬇️
pkg/fileutil/read_dir.go	91.3% <0%> (-8.7%)	⬇️
clientv3/leasing/cache.go	87.77% <0%> (-3.89%)	⬇️
etcdmain/grpc_proxy.go	59.92% <0%> (-2.69%)	⬇️
clientv3/balancer/balancer.go	84.49% <0%> (-2.33%)	⬇️
etcdserver/api/v2http/client.go	85.51% <0%> (-1.21%)	⬇️
etcdserver/server.go	74.53% <0%> (-0.51%)	⬇️
etcdserver/api/membership/cluster.go	77.5% <0%> (-0.46%)	⬇️
clientv3/watch.go	90.92% <0%> (-0.22%)	⬇️
etcdserver/api/v3election/election.go	66.66% <0%> (ø)	⬆️
... and 14 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 cc08c1b...f17c038. Read the comment docs.

[gyuho]

Manually tested, and it passed with current master. LGTM. Thanks.