Manifest: don't use the test stanza

Context

Since we now generate dune files, using tests and test stanza has little value over being explicit.

test stanza can have counter intuitive behaviors. In particular, it doesn't take modes into account and always tries run the native binary TEST.exe which might no be available if modes only specify js.

This MR

• generate executable + runtest alias stanzas instead of using the test stanza
• remove action from maker, it was only valid for test stanza and only used when automatically creating the runtest alias
• simplify the manifest API, unifying test and test_exe into a single function with an addition optional argument ?runtest

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR