fix: add file:// prefix for imports in test runner

[bartlomieju]

Otherwise test runner is not working properly when requested from remote server:

// run in root of deno_std repo
$ deno -A https://deno.land/std/testing/runner.ts 
Download https://deno.land/Users/biwanczuk/dev/deno_std/strings/pad_test.ts
error: Uncaught TypeError: Import 'https://deno.land/Users/biwanczuk/dev/deno_std/strings/pad_test.ts' failed: 404 Not Found

It's caused because base for specifier is a remote script (https://deno.land/std/testing/runner.ts) and found test files are absolute paths (/Users/biwanczuk/dev/deno_std/strings/pad_test.ts).

I explicitly added file:// prefix to import, that should resolve the problem.

[ry]

LGTM
There’s another thread where people are discussing the validity of exactly this case... can’t find a link right now.. but you might want to chime in and mention this situation.