use a more readable name for creator into the tests

[Pantani]

          Can we use a more readable name here? like `signer` or `creator`

	<%= MsgSigner.LowerCamel %> := "Signer_A"

Originally posted by @Pantani in #3707 (comment)

[julienrbrt]

I need to touch those tests in #4058, so I'll be making more readable at the same time