Create mock for RemotExecutor interface with mockery

The following discussion from !1680 (merged) should be addressed:

• @steveazz started a discussion: (+1 comment)

  If we do it like below it will just create this mock_RemoteExecutor.go which should probably be used instead of the hand-rolled fakeRemoteExecutor

  	mockery $(MOCKERY_FLAGS) -dir=./executors/kubernetes -all -inpkg

  What do you think if inside this merge request we set it to the suggestion above, and then in a follow up merge request we remove the fakeRemoteExecutor, what do you think?

Proposal

• Update https://gitlab.com/gitlab-org/gitlab-runner/-/blob/ee80e237c50b733e4ec2afc879ee022d93bbb373/Makefile#L143 to generate mocks for all of executors/kubernetes
• Remove the use of fakeRemoteExecutor and use the generated mock instead.