[Metricbeat] Improve go test flags

[kaiyan-sheng]

When testing kubernetes metricbeat module, I realize we have three different flags for generating/updating files:

go test -generate
go test -data
go test -update_expected

For -generate, it regenerates the .json files under _meta/testdata. For -update_expected, it regenerates the .expected files under _meta/test. For this enhancement, the goal is to use one flag -generate to generate/update all files. So -data and -update_expected can be deprecated later.

It would be nice to have a common package for all kinds of test helpers so they reuse the logic to decide if files should be regenerated.

[jsoriano]

-data is documented in the devguide and in the checklist of the issue template for new modules, so I would go for it, and deprecate the others.

[mtojek]

PRs merged, resolving.