Compile error test cases can now be given as a sequence of files:
  - "foo.1.zig"
  - "foo.2.zig"
  - "foo.3.zig"
  - etc.

This sequence of files is tested as incremental compilation updates to a
single "foo.zig" source file.

To help avoid mistakes, we enforce strict ordering for these files.
"foo.zig" cannot co-exist with "foo.X.zig", the sequence must include
"foo.1.zig", and no numbers may be skipped.