Make sure to actually use yarn to install the main-packages, otherwise the packages.json#resolutions property will not be used and @babel/helpers would not get overruled

Overruled @babel/helpers to fix how initializers play with decorated properties. Thus circumventing the imperformant and lengthy code being generated by babel in the non-legacy option.

packages/babel-helpers/test/fixtures/dependencies/multiple/output.js

@@ -0,0 +1,7 @@
+function _$_multiple_main() { return _$_multiple_dependency() + _$_multiple_dependency2(); }
+
+function _$_multiple_dependency2() { 1; }
+
+function _$_multiple_dependency() { 0; }
+
+_$_multiple_main;