Docs page on testing

[jcrist]

There's been some FUD messaging by other projects against msgspec as an unsafe C project. While it's written in C, safety is a spectrum and we're definitely not careless about security issues here. It would be good to write up a docs page on our testing setup to hopefully allay safety concerns. We should cover:

• Our high test coverage (full coverage in critical sections of the parsers)
• Our fuzz testing setups
• Our use of sanitizer tooling
• Our track record. Some purportedly safe rust projects make enough incorrect use of unsafe to not actually guarantee safety, leading to a number of segfault issues over the years.

This is not to minimize the benefit a safe language like rust provides (rust is good! and C does have footguns). But all C code is not the same and this project should definitely not be treated as "definitely unsafe, don't touch". All software is tradeoffs - if you need the performance benefits msgspec provides and can tolerate a "we're trying our best using the tooling available, no CVEs so far" level of guarantee, then maybe msgspec is a tool worth considering.