Allow `(alias ...)` in test(s) stanza to attach the test to a different alias, e.g. `longtests`

Desired Behavior

When (alias ...) is present in a test(s) stanza, run the test (and diff) for this alias and not the runtest alias.

This would allow to use nice concise test stanzas, including the diffing functionality, for the use case of running long tests / benchmarks only occasionnally (or in CI).

When one just wants to run some executable for some alias, it remains reasonably concise even without the test stanza : an executable stanza + a short rule stanza. With the diffing + promotion added, that's one more stanza with mostly boilerplate.

Example

I would like to be able to write:

(test
  (alias longtests)
  (name test)
  (libraries ...)
  (deps ...)
)

Instead of having to write

(executable
  (name test)
  (libraries ...)
)

(rule
  (deps ...)
  (action (with-stdout-to test.output (run ./test.exe)))
)

(rule
  (alias longtests)
  (action (diff test.expected test.output))
)

This would also make it easier to switch a test already using a test stanza to another alias.

Thank you very much for Dune,
Best.

The issue names no files or tests. Start by tracing how test(s) stanzas currently attach to the runtest alias, then examine alias handling and diff/promotion behavior; done means (alias longtests) runs the test and diff for longtests instead of runtest without the extra executable and rule boilerplate.