Issue #3025685 by quietone: Add error msg to assertions in MigrateSourceTestBase

core/modules/migrate/tests/src/Kernel/MigrateSourceTestBase.php

@@ -175,13 +175,14 @@ public function testSource(array $source_data, array $expected_data, $expected_c
       foreach ($expected as $key => $value) {
         $this->assertArrayHasKey($key, $actual);
 
+        $msg = sprintf("Value at 'array[%s][%s]' is not correct.", $i - 1, $key);
         if (is_array($value)) {
           ksort($value);
           ksort($actual[$key]);
-          $this->assertEquals($value, $actual[$key]);
+          $this->assertEquals($value, $actual[$key], $msg);
         }
         else {
-          $this->assertSame((string) $value, (string) $actual[$key]);
+          $this->assertEquals((string) $value, (string) $actual[$key], $msg);
         }
       }
     }