test(grid): Fix failing tests

Author: MPopov

projects/igniteui-angular/src/lib/core/styles/components/grid/_grid-theme.scss

@@ -591,7 +591,7 @@
     ), map-get($theme, variant));
 
     $grid-header-weight: map-get((
-        material: 400,
+        material: 600,
         fluent: 800
     ), map-get($theme, variant));