mcs add tests for groups and mock same function twice

[cesnietor]

Add test for function that uses the same function to mock twice.
basically it justs uses a flag:

// groupUpdate uses getInfo() twice which uses getGroupDescription() so we need to mock as if it called
	// the function twice but the second time returned an error
	is2ndRunGroupInfo := false
	// mock function response from updateGroupMembers()
	minioGetGroupDescriptionMock = func(group string) (*madmin.GroupDesc, error) {
		if is2ndRunGroupInfo {
			return mockResponseAfterUpdate, nil
		}
		is2ndRunGroupInfo = true
		return mockResponseBeforeUpdate, nil
	}

[bexsoft]

LGTM