WIP: Fix TestTransportParseReference on macOS

Originally, when the test was written, an empty runRoot or graphRoot
was rejected by storage.GetStore.

That changed in ce62453 , when
GetStore started accepting inputs with at least one of runRoot / graphRoot
set, using the default path for the unspecified one.

The test continued to work because the default value, on macOS,
was /run/..., and an attempt to create that directory failed.

The recent c/storage configuration redesign changed the defaults,
and they are now a temporary runtime dir in $TMPDIR.

So, update the test to match.  Explicitly specify the graph driver
to avoid interfering with other local storage.

Signed-off-by: Miloslav Trmač <mitr@redhat.com>

Author: mtrmac

image/storage/storage_transport_test.go

@@ -100,16 +100,16 @@ func TestTransportParseReference(t *testing.T) {
 	root := store.GraphRoot()
 
 	for _, c := range []struct{ prefix, expectedDriver, expectedRoot, expectedRunRoot string }{
-		{"", driver, root, ""},                                             // Implicit store location prefix
-		{"[unterminated", "", "", ""},                                      // Unterminated store specifier
-		{"[]", "", "", ""},                                                 // Empty store specifier
-		{"[relative/path]", "", "", ""},                                    // Non-absolute graph root path
-		{"[" + driver + "@relative/path]", "", "", ""},                     // Non-absolute graph root path
-		{"[@" + root + "suffix2]", "", "", ""},                             // Empty graph driver
-		{"[" + driver + "@]", "", "", ""},                                  // Empty root path
-		{"[thisisunknown@" + root + "suffix2]", "", "", ""},                // Unknown graph driver
-		{"[" + root + "suffix1]", "", "", ""},                              // A valid root path, but no run dir
-		{"[" + driver + "@" + root + "suffix3+relative/path]", "", "", ""}, // Non-absolute run dir
+		{"", driver, root, ""},                                                 // Implicit store location prefix
+		{"[unterminated", "", "", ""},                                          // Unterminated store specifier
+		{"[]", "", "", ""},                                                     // Empty store specifier
+		{"[relative/path]", "", "", ""},                                        // Non-absolute graph root path
+		{"[" + driver + "@relative/path]", "", "", ""},                         // Non-absolute graph root path
+		{"[@" + root + "suffix2]", "", "", ""},                                 // Empty graph driver
+		{"[" + driver + "@]", "", "", ""},                                      // Empty root path
+		{"[thisisunknown@" + root + "suffix2]", "", "", ""},                    // Unknown graph driver
+		{"[" + driver + "@" + root + "suffix1]", driver, root + "suffix1", ""}, //A valid root path, but no run dir
+		{"[" + driver + "@" + root + "suffix3+relative/path]", "", "", ""},     // Non-absolute run dir
 		{
 			"[" + driver + "@" + root + "suffix3+" + root + "suffix4]",
 			driver,