cli/command/system: TestEventsFormat: set cmd.Args to prevent test-failures

[thaJeztah]

• similar to test spring-cleaning #5224

When running the tests with options set, such as -update for updating "golden" files, this test would pick up test arguments because no arguments were set to invoke the command;

go test . -update
Error: unknown shorthand flag: 'u' in -update
Usage:
  events [OPTIONS] [flags]

Flags:
  -f, --filter filter   Filter output based on conditions provided
      --format string   Format output using a custom template:
                        'json':             Print in JSON format
                        'TEMPLATE':         Print output using the given Go template.
                        Refer to https://docs.docker.com/go/formatting/ for more information about formatting output with templates
  -h, --help            help for events
      --since string    Show all events created since timestamp
      --until string    Stream events until this timestamp

--- FAIL: TestEventsFormat (0.00s)
    --- FAIL: TestEventsFormat/default (0.00s)
        events_test.go:75: assertion failed: error is not nil: unknown shorthand flag: 'u' in -update
    --- FAIL: TestEventsFormat/json (0.00s)
        events_test.go:75: assertion failed: error is not nil: unknown shorthand flag: 'u' in -update
    --- FAIL: TestEventsFormat/json_template (0.00s)
        events_test.go:75: assertion failed: error is not nil: unknown shorthand flag: 'u' in -update
    --- FAIL: TestEventsFormat/json_action (0.00s)
        events_test.go:75: assertion failed: error is not nil: unknown shorthand flag: 'u' in -update

This patch:

• changes the test to use command-arguments instead of manually setting the flag options; this also adds test-coverage for parsing actual command arguments.
• discards stdout/stderr of the command to prevent noise in test output

- A picture of a cute animal (not mandatory but encouraged)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.51%. Comparing base (91d097e) to head (73ff81b).
Report is 12 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5695   +/-   ##
=======================================
  Coverage   59.51%   59.51%           
=======================================
  Files         346      346           
  Lines       29379    29379           
=======================================
  Hits        17486    17486           
  Misses      10923    10923           
  Partials      970      970           

[thaJeztah]

Failures are due to the docker / docker:dind image on hub being broken; docker-library/docker#520