Further tweak :test dependency handling

- ensure that cases where `:build` and `:test` both apply to a
  dependency that we handle it appropriately
- fix reintroduced regression
  (https://github.com/Homebrew/brew/pull/6471#issuecomment-533414923).
  Fixes #6470.

Library/Homebrew/build.rb

@@ -52,6 +52,8 @@ class Build
         Requirement.prune
       elsif req.prune_if_build_and_not_dependent?(dependent, formula)
         Requirement.prune
+      elsif req.test?
+        Requirement.prune
       end
     end
   end
@@ -65,6 +67,8 @@ class Build
         Dependency.prune
       elsif dep.build?
         Dependency.keep_but_prune_recursive_deps
+      elsif dep.test?
+        Dependency.prune
       end
     end
   end

Library/Homebrew/formula_installer.rb

@@ -488,10 +488,11 @@ class FormulaInstaller
 
       if dep.prune_from_option?(build)
         Dependency.prune
-      elsif dep.test? && !dep.build? && !include_test?
-        Dependency.prune
-      elsif dep.build? && !dep.test? && install_bottle_for?(dependent, build)
-        Dependency.prune
+      elsif dep.test? || (dep.build? && install_bottle_for?(dependent, build))
+        keep = false
+        keep ||= dep.test? && include_test? && dependent == formula
+        keep ||= dep.build? && !install_bottle_for?(dependent, build)
+        Dependency.prune unless keep
       elsif dep.prune_if_build_and_not_dependent?(dependent)
         Dependency.prune
       elsif dep.satisfied?(inherited_options[dep.name])