[fuchsia] Permit relative entries to the fuchsia_tester (#28821)
[fuchsia] Permit relative entries to the fuchsia_tester (#28821)

This change permits
(1) a relative --flutter-shell
(2) relative (source, dill) entries within the test JSON file

and enables portability in the invocation scripts produced by fuchsia's
dart test GN template.
cbracken/flutter,Hixie/flutter,Hixie/flutter,tvolkert/flutter,flutter/flutter,cbracken/flutter,Hixie/flutter,flutter/flutter,tvolkert/flutter,flutter/flutter,flutter/flutter,Hixie/flutter,flutter/flutter,tvolkert/flutter,cbracken/flutter,Hixie/flutter,tvolkert/flutter,Hixie/flutter,tvolkert/flutter,Hixie/flutter,tvolkert/flutter,cbracken/flutter,flutter/flutter,flutter/flutter,cbracken/flutter,tvolkert/flutter,tvolkert/flutter,cbracken/flutter,flutter/flutter,tvolkert/flutter,cbracken/flutter,Hixie/flutter,cbracken/flutter,tvolkert/flutter,cbracken/flutter,Hixie/flutter,flutter/flutter,cbracken/flutter,flutter/flutter,Hixie/flutter