fix: better paths handling in test runner

[bartlomieju]

Fixes #570
Fixes #569

[bartlomieju]

deno -A ./testing/runner.ts https://deno.land/std/fmt/sprintf_test.ts
Compile file:///Users/biwanczuk/dev/deno_std/testing/mod.ts
Found 1 matching test files.
running 29 tests
OK     noVerb (0.34ms)
OK     percent (0.09ms)
OK     testBoolean (0.43ms)
OK     testIntegerB (0.62ms)
OK     testIntegerC (0.14ms)
OK     testIntegerD (0.11ms)
OK     testIntegerO (0.11ms)
OK     testIntegerx (0.26ms)
OK     testIntegerX (0.10ms)
OK     testFloate (0.37ms)
OK     testFloatE (0.15ms)
OK     testFloatfF (0.46ms)
OK     testString (0.09ms)
OK     testHex (0.07ms)
OK     testHeX (0.06ms)
OK     testType (0.08ms)
OK     testPositional (0.13ms)
OK     testSharp (0.13ms)
OK     testWidthAndPrecision (0.23ms)
OK     testDash (0.07ms)
OK     testPlus (0.08ms)
OK     testSpace (0.07ms)
OK     testZero (0.06ms)
OK     testThorough (2.15ms)
OK     testWeirdos (0.10ms)
OK     formatV (0.50ms)
OK     formatJ (0.08ms)
OK     flagLessThan (0.23ms)
OK     testErrors (0.33ms)

test result: OK 29 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out (10.81ms)

[nayeemrmn]

Can you add -S to the shebang? Ref #545.

[ry]

LGTM