move tephi/tests to tests

[ESadek-MO]

part of #164

[pp-mo]

Warning ?

I think it's fair to say that we are having second thoughts on whether tests should be part of the package.

It had been raised in the past that including the tests made the Iris checkout + install rather bloated.
Since then we put the tests outside the package in both geovista, and ncdata.

But this approach has been found to have a few problems of its own :

• tests can't be used to check install success
• users can't (so easily) run the tests
• the import of test submodules can be tricky, especially relative imports, as the tests aren't in a proper package
  • (N.B. pytest has clever ways of managing this, but these can also have drawbacks)