Replace for_each_index usage in tests

[dsharlet]

The use of for_each_index in tests might be causing a few problems:

• It generates a lot of complicated code for the compiler to generate. pipeline_test.cc takes a long time to compile with --config=asan.
• Somehow, when an assert fails in a lambda, it seems to only unwind one level of the stack. Inside a callback like for_each_index, this means you can get thousands of assert failure messages, when the test should just fail on the first one.
• It's hard to debug these tests without being able to look at the buffer data somehow.

We should replace these with something like:

template <typename T>
bool is_buffer_equal_to(const slinky::buffer<T>& a, std::function<T(std::span<const index_t>)>b);

This would allow comparing a buffer to some generated ground truth. We can add some ability to print out slices, and avoid asserting thousands of times.