refactor: test utils functions by removing redundant test instructions parsing and simplifying asserting functions

[chavacava]

Avoids parsing instructions twice and simplifies signature of asserting functions

1. Original code parsed test instructions twice:

	if parseInstructions(t, fullFilePath, src) == nil {   // first parsing
		assertSuccess(t, baseDir, stat, []lint.Rule{rule}, c)
		return
	}
	assertFailures(t, baseDir, stat, src, []lint.Rule{rule}, c) // second parsing inside assertFailures

2. Functions assertSuccess and assertFailures both took baseDir string, fi os.FileInfo as arguments to do exactly the same file path reconstruction that callers already do. Simplified both to receive the file path.