test: Don't panic if some tests failed
test: Don't panic if some tests failed

This commit removes the call to `panic!("Some tests failed")` at the end of all
tests run when running with libtest. The panic is replaced with
`std::process::exit` to have a nonzero error code, but this change both:

1. Makes the test runner no longer print out the extraneous panic message at the
   end of a failing test run that some tests failed. (this is already summarized
   in the output of the test run).
2. When running tests with `RUST_BACKTRACE` set it removes an extraneous
   backtrace from the output (only failing tests will have their backtraces in
   the output.

dwillmer/rust,graydon/rust,GBGamer/rust,dwillmer/rust,graydon/rust,GBGamer/rust,graydon/rust,aidancully/rust,aidancully/rust,dwillmer/rust,aidancully/rust,dwillmer/rust,dwillmer/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,aidancully/rust,dwillmer/rust,GBGamer/rust,GBGamer/rust,graydon/rust,graydon/rust,aidancully/rust,dwillmer/rust,dwillmer/rust,graydon/rust,GBGamer/rust