[mojo-stdlib] Replace tests using FileCheck with testing module where possible

[JoeLoser]

Review Mojo's priorities

• I have read the roadmap and priorities and I believe this request falls within the priorities.

What is your request?

Many tests use FileCheck still which isn't great for many reasons for a standard library. We should move these over to use functions like assert_equal, assert_true, and friends from the testing module where possible.

What is your motivation for this change?

Moving things over to use our testing module helps "eat our own cooking" as we build out the test framework. Additionally, often, the code is much shorter — while still achieving the same intent as the original code.

Any other details?

There are several test files still using FileCheck as can be identified with:

git grep -l '| FileCheck'

I wouldn't worry too much about the examples to start with; in fact, I recommend we fix those last only after we've changed as many of the standard library tests as we can.

Note: if multiple people plan on working on this, please coordinate to avoid two people working in porting the tests over for the same file.