Improve how caller context is set in tests

Instead of:

```
it('is some test', () => {
    caller = OWNER;
    contract.foo(arg1, arg2, caller);
})
```

we should do something like this:

```
it('is some test', () => {
    contract.set_caller(OWNER);
    contract.foo(arg1, arg2);
})
```

or even better:

```
it('is some test', () => {
    testConfig.set_caller(OWNER);
    contract.foo(arg1, arg2);
})
```