e2e/framework: absolute test file paths

Tests might want to use there own options for specifying a file path,
while still using the abstract file access API. For example,
framework.CreateFromManifests might be used to create a mixture of
files under the repo root and from elsewhere. To support this,
absolute paths can now be given to the testfiles package and they will
be read directly.

test/e2e/framework/testfiles/testfiles.go

@@ -129,9 +129,16 @@ type RootFileSource struct {
 }
 
 // ReadTestFile looks for the file relative to the configured
-// root directory.
+// root directory. If the path is already absolute, for example
+// in a test that has its own method of determining where
+// files are, then the path will be used directly.
 func (r RootFileSource) ReadTestFile(filePath string) ([]byte, error) {
-	fullPath := filepath.Join(r.Root, filePath)
+	var fullPath string
+	if path.IsAbs(filePath) {
+		fullPath = filePath
+	} else {
+		fullPath = filepath.Join(r.Root, filePath)
+	}
 	data, err := ioutil.ReadFile(fullPath)
 	if os.IsNotExist(err) {
 		// Not an error (yet), some other provider may have the file.