Made SimpleTestCase client attributes cached properties.

[adamchainz]

This avoids instantiating them in test cases that don't need them.

[adamchainz]

The saving here is pretty small - about 3 microseconds per test (that uses neither client):

In [2]: %timeit Client(), AsyncClient()
3.36 µs ± 28.8 ns per loop (mean ± std. dev. of 7 runs, 100000 loops each)

That can add up though, and it could be larger in custom test client classes.

[nessita]

Hello @adamchainz, quick question, shouldn't we provide setters for these new properties to ensure this change is backwards compatible? Thanks!

[adamchainz]

I think we can close this one for now, it's not a huge optimization.