Add assertIndeterminate assertion for checkbox. (#986)

* Add `assertIndeterminate` assertion for checkbox.

Reference: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/input/checkbox#indeterminate_state_checkboxes

Signed-off-by: Mior Muhammad Zaki <[email protected]>

* wip

Signed-off-by: Mior Muhammad Zaki <[email protected]>

* Update MakesAssertions.php

Co-authored-by: Taylor Otwell <[email protected]>

Author: crynobone

src/Concerns/MakesAssertions.php

@@ -486,6 +486,26 @@ public function assertNotChecked($field, $value = null)
         return $this;
     }
 
+    /**
+     * Assert that the given checkbox is in an indeterminate state.
+     *
+     * @param  string  $field
+     * @param  string|null  $value
+     * @return $this
+     */
+    public function assertIndeterminate($field, $value = null)
+    {
+        $this->assertNotChecked($field, $value);
+
+        PHPUnit::assertSame(
+            'true',
+            $this->resolver->findOrFail($field)->getAttribute('indeterminate'),
+            "Checkbox [{$field}] was not in indeterminate state."
+        );
+
+        return $this;
+    }
+
     /**
      * Assert that the given radio field is selected.
      *