[WIP] Fix: registry login test failing on OSX.

[dblock]

(Go noob here, sorry)

I am seeing this failure locally on OSX in integration tests.

$ go test -v -mod=vendor ./integration -run TestRun/doctl/registry/#
=== RUN   TestRun
Suite: doctl
Total: 385 | Focused: 0 | Pending: 0
=== RUN   TestRun/doctl
=== RUN   TestRun/doctl/registry/#/all_required_flags_are_passed/writes_a_docker_config.json_file
=== PAUSE TestRun/doctl/registry/#/all_required_flags_are_passed/writes_a_docker_config.json_file
=== RUN   TestRun/doctl/registry/#/expiry-seconds_flag_is_passed/add_the_correct_query_parameter
=== PAUSE TestRun/doctl/registry/#/expiry-seconds_flag_is_passed/add_the_correct_query_parameter
=== RUN   TestRun/doctl/registry/#/read-only_flag_is_passed_and_the_token_doesn't_expire/add_the_correct_query_parameter
=== PAUSE TestRun/doctl/registry/#/read-only_flag_is_passed_and_the_token_doesn't_expire/add_the_correct_query_parameter
=== CONT  TestRun/doctl/registry/#/all_required_flags_are_passed/writes_a_docker_config.json_file
=== CONT  TestRun/doctl/registry/#/read-only_flag_is_passed_and_the_token_doesn't_expire/add_the_correct_query_parameter
=== CONT  TestRun/doctl/registry/#/expiry-seconds_flag_is_passed/add_the_correct_query_parameter
    registry_login_test.go:123: 
        	Error Trace:	/Users/dblock/source/doctl/dblock-doctl/integration/registry_login_test.go:123
        	            				/Users/dblock/source/doctl/dblock-doctl/vendor/github.com/sclevine/spec/spec.go:269
        	            				/Users/dblock/source/doctl/dblock-doctl/vendor/github.com/sclevine/spec/spec.go:262
        	            				/Users/dblock/source/doctl/dblock-doctl/vendor/github.com/sclevine/spec/parser.go:133
        	Error:      	Received unexpected error:
        	            	exit status 1
        	Test:       	TestRun/doctl/registry/#/expiry-seconds_flag_is_passed/add_the_correct_query_parameter
        	Messages:   	Logging Docker in to registry.digitalocean.com
        	            	Error: error storing credentials - err: exit status 1, out: `The specified item already exists in the keychain.`


Passed: 2 | Failed: 1 | Skipped: 0

--- FAIL: TestRun (0.46s)
    --- FAIL: TestRun/doctl (0.00s)
        --- FAIL: TestRun/doctl/registry/#/expiry-seconds_flag_is_passed/add_the_correct_query_parameter (0.45s)
        --- PASS: TestRun/doctl/registry/#/all_required_flags_are_passed/writes_a_docker_config.json_file (0.46s)
        --- PASS: TestRun/doctl/registry/#/read-only_flag_is_passed_and_the_token_doesn't_expire/add_the_correct_query_parameter (0.46s)
FAIL
FAIL	github.com/digitalocean/doctl/integration	1.630s
FAIL

This looks like a side-effect of a prior test that successfully stores credentials and doesn't clean up.

I tried to do a registry logout in the test but haven't succeeded in making it work. If someone can help me write that I can finish this PR.