Intermittent unit test failure for sidecar DNS

[zcahana]

There's a unit test for the sidecar DNS function that intermittently fails (with a panic), thus failing the Travis build. The failure does seems to be related to a code bug, but rather to a test bug where several unit tests are attempting to bind a DNS server to the same server port:
A potential solution can be to allocate sequential, exclusive ports to each unit tests.

Example failure (source):

=== RUN   TestRequestsSRVWithTag
time="2016-12-12T09:55:30Z" level=info msg="Starting DNS server" 
time="2016-12-12T09:55:30Z" level=error msg="Error starting DNS server" error="listen udp :8823: bind: address already in use" 
time="2016-12-12T09:55:32Z" level=info msg="Shutting down DNS server" 
time="2016-12-12T09:55:32Z" level=error msg="Error shutting down DNS server" error="dns: server not started" 
--- FAIL: TestRequestsSRVWithTag (2.20s)
	Error Trace:	server_test.go:207
	Error:		Received unexpected error read udp 127.0.0.1:53017->127.0.0.1:8823: i/o timeout
		
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x488c2f]
goroutine 11 [running]:
panic(0x777100, 0xc4200121b0)
	/home/travis/.gimme/versions/go1.7.1.linux.amd64/src/runtime/panic.go:500 +0x1a1
testing.tRunner.func1(0xc420082600)
	/home/travis/.gimme/versions/go1.7.1.linux.amd64/src/testing/testing.go:579 +0x25d
panic(0x777100, 0xc4200121b0)
	/home/travis/.gimme/versions/go1.7.1.linux.amd64/src/runtime/panic.go:458 +0x243
github.com/amalgam8/amalgam8/sidecar/dns.(*TestSuite).TestRequestsSRVWithTag(0xc42001aa50)
	/home/travis/gopath/src/github.com/amalgam8/amalgam8/sidecar/dns/server_test.go:208 +0x12f
reflect.Value.call(0xc4201f7200, 0xc4200325a8, 0x13, 0x7d1e26, 0x4, 0xc420053f40, 0x1, 0x1, 0x5d3a48, 0x7ceee0, ...)
	/home/travis/.gimme/versions/go1.7.1.linux.amd64/src/reflect/value.go:434 +0x5c8
reflect.Value.Call(0xc4201f7200, 0xc4200325a8, 0x13, 0xc420053f40, 0x1, 0x1, 0xc4203b6400, 0x8b0, 0x8c)
	/home/travis/.gimme/versions/go1.7.1.linux.amd64/src/reflect/value.go:302 +0xa4
github.com/amalgam8/amalgam8/vendor/github.com/stretchr/testify/suite.Run.func2(0xc420082600)
	/home/travis/gopath/src/github.com/amalgam8/amalgam8/vendor/github.com/stretchr/testify/suite/suite.go:95 +0x1cb
testing.tRunner(0xc420082600, 0xc420297c70)
	/home/travis/.gimme/versions/go1.7.1.linux.amd64/src/testing/testing.go:610 +0x81
created by testing.(*T).Run
	/home/travis/.gimme/versions/go1.7.1.linux.amd64/src/testing/testing.go:646 +0x2ec