Integration: Test flakiness fixes

[ptabor]

• Fixed 2 cases of leaking go-routines from core-logic (e.g. embed was not stopping HTTP server)
• Enforced BeforeTest in all integrational tests.
• TestBalancerUnderNetworkPartitionWatch* are not picking server they talk to semi-randomly, so are guaranteed to be connected to 'separated' node to confirm WithLeader behavior of watch.

[codecov-io]

Codecov Report

Merging #12782 (6657d59) into master (e599f4a) will decrease coverage by 5.86%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master   #12782      +/-   ##
==========================================
- Coverage   72.24%   66.37%   -5.87%     
==========================================
  Files         427      404      -23     
  Lines       33159    31946    -1213     
==========================================
- Hits        23955    21205    -2750     
- Misses       7271     8802    +1531     
- Partials     1933     1939       +6     

Flag	Coverage Δ
all	66.37% <83.33%> (-5.87%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
server/embed/etcd.go	75.98% <66.66%> (-0.12%)	⬇️
etcdctl/snapshot/v3_snapshot.go	62.91% <100.00%> (+0.17%)	⬆️
raft/quorum/quorum.go	0.00% <0.00%> (-100.00%)	⬇️
raft/tracker/state.go	0.00% <0.00%> (-100.00%)	⬇️
pkg/tlsutil/cipher_suites.go	0.00% <0.00%> (-100.00%)	⬇️
pkg/transport/sockopt_unix.go	0.00% <0.00%> (-100.00%)	⬇️
...ver/proxy/grpcproxy/adapter/lock_client_adapter.go	0.00% <0.00%> (-100.00%)	⬇️
pkg/report/report.go	0.00% <0.00%> (-95.58%)	⬇️
pkg/srv/srv.go	0.00% <0.00%> (-87.94%)	⬇️
pkg/report/timeseries.go	0.00% <0.00%> (-86.77%)	⬇️
... and 140 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 e599f4a...6657d59. Read the comment docs.

[wpedrak]

LGTM