tests: Avoid dealing with character encoding issues on command line.

tests: Avoid dealing with character encoding issues on command line.

Unix-like systems don't have a command-line encoding, so we can pass
whatever bytes we want.  mingw uses some particular encoding for
command line arguments, so this screws up trying to pass UTF-8 in.  This
commit side-steps the problem by passing only ASCII characters on the
command line and making the test program decode the bytes.

This fixes several test failures under mingw.