Add a content keyword to file_test, `[[@0xAB]]`, that expands to the
code unit 0xAB, and use that instead of putting raw malformed code units
in test files.
Instead of printing the raw input bytes in snippets in diagnostics,
replace non-printable characters with <AB> in the output, being careful
to still compute the location of the caret and underscore properly.
This opens the door for replacing all `library ...` lines in toolchain
test files with `library "[[@TEST_NAME]]";`. That's technically more
typing in a lot of cases, but OTOH means we can just do some copy-paste
boilerplate and stop carefully writing library names.
Also cleans up the test setup, because it's getting messy. I'm trying to
make it easier to see the divisions of tests and the output associated
with them. StringSwitch looked like a way to do this, with a few edits
to make it work nicely.
Richard Smith
Jon Ross-Perkins