Avoid un-used variables when depending on fixtures w/ side effects.

[obmarg]

Sometimes you might want to depend on a fixture that has side-effects, but you don't actually need to use that fixture in your code. Currently if you do this, you'll get a compilation warning saying a variable is un-used.

It might be nice to provide a work-around for this. Either:

• We could just add no-op expressions that make use of all dependencies to a fixture in it's create function.
• We could permit underscore prefixes on arguments to fixtures to denote that the variable is un-used. These would still resolve to the non-underscore prefixed name. If we did this we'd probably need to explicitly ban fixtures with an underscore prefix, to avoid clashes & confusion.

[obmarg]

36 could take care of/vastly simplify this.

[obmarg]

41 made this possible by defining a separate function and registering it with register_fixture.