Allow to customize runtest aliases for inline_tests and tests stanza

[hhugo]

Desired Behavior

I'd like to be able to customize the runtest alias when using (inline_tests) in a library stanza or (tests ..).
I'd like to customize this alias per modes.
The goal is to run js tests only when node is installed / available.

Example

(library
  (name lib)
  (inline_tests (modes (js (alias runtest-js)) (native (alias runtest-native)))))

(tests
   (names a b c)
   (modes (js (alias runtest-js)) (native (alias runtest-native))))

related to #4951

dune build @runtest-js
dune build @runtest-native