Test suite: Change @asserts to errors, and change bare using Foo to using Foo: name, anothername, ...

[DilumAluthge]

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[DilumAluthge]

Okay, CI looks good.

[DilumAluthge]

@kleinhenz This one is ready for review as well. The basic idea is to implement a few best practices:

1. In the docs for @assert, the Julia manual says, "an assert might be disabled at some optimization levels". So in this PR, I change the @asserts to explicit errors, because we never want these tests to be disabled, regardless of the optimization level.
2. As described in JuliaLang/julia#42080, it's not ideal to do a bare using Foo. It's better (and more future-proof) to do either using Foo: hello, world, blah or import Foo. So in this PR, I remove uses of bare using Foo.
3. I also mark as const a few variables that we expect to never change.