Refactor Makefile: only list the spec names once

[hwwhww]

Issue

Every time I added a new spec, I forget to add it to the linter and test coverage scope.

How did I fix it

Add a list ALL_EXECUTABLE_SPECS = phase0 altair bellatrix capella deneb. So we only have to update this variable for future specs.

Note: we still have to manually update MARKDOWN_FILES content because the paths differ.

[djrtwo]

nice!

can we not do the specs by wild-carding the sub-dir even for specs without it?

[hwwhww]

I just found that MARKDOWN_FILES is only for check_toc and all the files under specs are ToC-checked. So we can wild-card all files under specs now.