feat(builtin): remove lean executable as a dependency for builtin .lean files

Otherwise, we have to rebuild all .lean files whenever we change the executable.
This commit also adds a test for each .lean file.
This is useful for increasing coverage and having a log on how long does it take to process these files.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(builtin): remove lean executable as a dependency for builtin .lean files