Merge all assertFormatterRdfa() unit test calls into one single unit test

Problem/Motivation

This is a cleanup issue for something that was pointed by alexpott and webchick. Each Drupal unit test still takes some time to bootstrap Drupal, even if it's not as long as a full install.

+++ b/core/modules/rdf/lib/Drupal/rdf/Tests/Field/EmailFieldRdfaTest.php
@@ -0,0 +1,64 @@
+  /**
+   * Tests the plain formatter.
+   */
+  public function testPlainFormatter() {
+    $this->assertFormatterRdfa('text_plain', 'http://schema.org/email', $this->testValue);
+  }
+
+  /**
+   * Tests the mailto formatter.
+   */
+  public function testMailToFormatter() {
+    $this->assertFormatterRdfa('email_mailto', 'http://schema.org/email', $this->testValue);
+  }

Proposed resolution

+++ b/core/modules/rdf/lib/Drupal/rdf/Tests/Field/EmailFieldRdfaTest.php
@@ -0,0 +1,64 @@
+  /**
+   * Tests all the formatters.
+   */
+  public function testAllFormatters() {
+   // Tests the plain formatter.
+    $this->assertFormatterRdfa('text_plain', 'http://schema.org/email', $this->testValue);
+   // Tests the mailto formatter.
+    $this->assertFormatterRdfa('email_mailto', 'http://schema.org/email', $this->testValue);
+  }