Cherry-pick #7766 to 6.x: Add goTestUnit and goTestIntegration to magefile

[graphaelli]

Cherry-pick of PR #7766 to 6.x branch. Original message:

Consider these targets as incubating. They are not used by any of the Makefiles yet.
I added it to the Windows CI only to start testing it out and to resolve an issue where
compilation could fail and success would be reported by the powershell script.

$ mage -h goTestUnit
mage gotestunit:

GoTestUnit executes the Go unit tests.
Use TEST_COVERAGE=true to enable code coverage profiling.
Use RACE_DETECTOR=true to enable the race detector.

$ TEST_COVERAGE=true RACE_DETECTOR=true mage goTestUnit
>> go test: Unit Testing
SUMMARY:
  Fail:     0
  Skip:     2
  Pass:     807
  Packages: 70
  Duration: 21.459313277s
  Coverage Report: /Users/akroh/go/src/github.com/elastic/beats/libbeat/build/TEST-go-unit.html
  JUnit Report:    /Users/akroh/go/src/github.com/elastic/beats/libbeat/build/TEST-go-unit.xml
  Output File:     /Users/akroh/go/src/github.com/elastic/beats/libbeat/build/TEST-go-unit.out
>> go test: Unit Test Passed

$ TEST_COVERAGE=true RACE_DETECTOR=true mage goTestUnit
>> go test: Unit Testing
FAILURES:
Package: github.com/elastic/beats/libbeat/processors
Test:    TestDemo
processor_test.go:36: Only failing tests are logged. But you can use 'mage -v goTestUnit'
	to see all of the go test output or just view the output file list in the summary.
----
SUMMARY:
  Fail:     1
  Skip:     2
  Pass:     807
  Packages: 70
  Duration: 21.53730358s
  Coverage Report: /Users/akroh/go/src/github.com/elastic/beats/libbeat/build/TEST-go-unit.html
  JUnit Report:    /Users/akroh/go/src/github.com/elastic/beats/libbeat/build/TEST-go-unit.xml
  Output File:     /Users/akroh/go/src/github.com/elastic/beats/libbeat/build/TEST-go-unit.out
>> go test: Unit Test Failed
Error: go test failed: 1 test failures
$ echo $?
1

[graphaelli]

this isn't right