To support this, add a mechanism for filtering test output to filter
out uninteresting diffs.
This adds some coverage for the code being changed in
https://github.com/WebAssembly/wasi-libc/pull/184.
wasi-libc builds special versions of printf and other routines optimized
for the case where long double and floating-point in general are not
needed. Ensure that everything still compiles and runs.
This adds a very primitive test harness and tests relevant to the recent
changes to how program startup works, as well as the upcoming changes
to support LTO.