build: add exception for missing MDC test (#22188)

Adds an exception for one missing test until #22187 is merged.

Author: crisbeto

scripts/check-mdc-tests-config.ts

@@ -140,6 +140,9 @@ export const config = {
           'element is inside an ngIf'
     ],
     'mdc-select': [
+      // TODO(crisbeto): remove this exception once #22187 lands.
+      'should float the label on focus if it has a placeholder',
+
       // These tests are excluded, because they're verifying the functionality that positions
       // the select panel over the trigger which isn't supported in the MDC select.
       'should set the width of the overlay based on a larger trigger width',