[CI] fix hardcode in TestImageHistory

[biubiubiuboomboomboom]

Motivation

Fix hardcoded CreatedSince values in TestImageHistory .

Related Issue

#4067

Change

• Replaced the fixed string value used in assert.Equal for the CreatedSince field with the value returned by the formatter.TimeSinceInHuman function in relevant unit test cases.

• Added a unit test case for the formatter.TimeSinceInHuman function .

Others

I noticed that other functions in formatter.go seem to have no unit tests . There may be a risk when these functions are changed .

[apostasie]

Thanks @biubiubiuboomboomboom

Two nits about using t.Parallel but otherwise looks good.