Change compile requirements for running cargo test

[YeungOnion]

Do we need the nightly compiler just for the test suite? The requirement is for feature fn_traits (I believe this requires feature unboxed_closures) is from statrs::testing_boiler but nowhere else.

Feedback seems to be,

• non-obvious to test with nightly for new contributors (even when familiar with rust projects)
• assertion error messages reference call to macro statrs::testing_boiler, not clear what line test is defined on.

Open to other feedback, but what I'm imagining is a few proc macros that expand to unwraps or assertions

• create(dist_family_name, tuple_of_dist_parameters, ...error msg)
• bad_create(... sim to previous asserting is_err
• test_almost(dist_family_name, tuple_of_dist_parameters, dot_method_call_with_args, ...error_msg)

[FreezyLemon]

Related: #232

I'm pretty sure the testing_boiler macro can be changed so that it doesn't need any nightly compiler features.

I wonder about the second part. Does RUST_BACKTRACE=1 not give any useful information? How would an optional error message, e.g. in test_almost(/* .. */, "error message here"), help with test suites like these? There must be a better way than adding error messages "line 1", "line 2", ..., right?

[YeungOnion]

RUST_BACKTRACE=1 does provide useful info, perhaps I'm a little silly. I think the messages would be convenient, but I think changing what we have for existing would be better.

[YeungOnion]

resolved by #234