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.
a91f19f356