yamlfmt_test: re-enable and fix doublestar tests

Doublestar tests have been disabled for quite a long time and I honestly
forget why they were. Regardless, they were missing some features to be
properly robust anyway. This PR re-enables the tests and adds two test
cases relevant to google#97 that will be skipped individually for now.

path_collector.go

@@ -2,7 +2,6 @@ package yamlfmt
 
 import (
 	"io/fs"
-	"log"
 	"os"
 	"path/filepath"
 	"strings"
@@ -116,13 +115,7 @@ func (c *DoublestarCollector) CollectPaths() ([]string, error) {
 		}
 		excluded := false
 		for _, pattern := range c.Exclude {
-			absPath, err := filepath.Abs(path)
-			if err != nil {
-				// I wonder how this could ever happen...
-				log.Printf("could not create absolute path for %s: %v", path, err)
-				continue
-			}
-			match, err := doublestar.PathMatch(filepath.Clean(pattern), absPath)
+			match, err := doublestar.PathMatch(filepath.Clean(pattern), path)
 			if err != nil {
 				return nil, err
 			}