avoid re-generating some bitmaps on test
move some test-generated files to temp dir
raco test: handling of spurious files

A `--drdr` run shouldn't stop because a discoevered file
disappears (such as one generated temporarily by a test).

Also, use new style for some errors.
raco test: add `--heartbeat` flag

Useful when running many tests in parallel to keep track of a test
that is running especially long (and maybe stuck).
bitwise-bit-field: repair fixnum overflow problems

Bug reported by Roman Klochkov