test(Form): add test cases of methods

src/form/test/__snapshots__/methods.spec.js.snap

@@ -0,0 +1,17 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`resetValidation method - reset one field 1`] = `
+<form class="van-form">
+  <div class="van-cell van-field">
+    <div class="van-cell__value van-cell__value--alone van-field__value">
+      <div class="van-field__body"><input type="text" name="A" class="van-field__control"></div>
+    </div>
+  </div>
+  <div class="van-cell van-field van-field--error">
+    <div class="van-cell__value van-cell__value--alone van-field__value">
+      <div class="van-field__body"><input type="text" name="B" class="van-field__control"></div>
+      <div class="van-field__error-message">B failed</div>
+    </div>
+  </div> <button type="submit" class="van-button van-button--default van-button--normal"></button>
+</form>
+`;