Test coverage for FieldTypeDefaults process plugin

Looks good but I have nits, as I often do...

1. +++ b/core/modules/field/tests/src/Unit/Plugin/migrate/process/d6/FieldTypeDefaultsTest.php
   @@ -0,0 +1,48 @@
   +   * Test that various default cases.
   +   *
   

   "Test that various default cases"? :)

2. +++ b/core/modules/field/tests/src/Unit/Plugin/migrate/process/d6/FieldTypeDefaultsTest.php
   @@ -0,0 +1,48 @@
   +    $this->assertNull($plugin->transform(NULL, $this->migrateExecutable, $this->row, 'property'));
   +    $this->assertEquals('string', $plugin->transform('string', $this->migrateExecutable, $this->row, 'property'));
   +    $this->assertEquals(1234, $plugin->transform(1234, $this->migrateExecutable, $this->row, 'property'));
   +    $this->assertEquals('datetime_default', $plugin->transform([], $this->migrateExecutable, $this->row, 'property'));
   

   I'd like to see a few comments here explaining the input and why the expected output is expected.

3. +++ b/core/modules/field/tests/src/Unit/Plugin/migrate/process/d6/FieldTypeDefaultsTest.php
   @@ -0,0 +1,48 @@
   +  /**
   +   * @covers ::transform
   +   */
   +  public function testDefaultsException() {
   

   There's no explanatory line in the doc comment...

4. +++ b/core/modules/field/tests/src/Unit/Plugin/migrate/process/d6/FieldTypeDefaultsTest.php
   @@ -0,0 +1,48 @@
   +    $plugin = new FieldTypeDefaults([], 'field_type_defaults', []);
   

   This is repeated in testDefaults(); can it go in setUp()?

From a recent learning experience with jhodgdon I've learned that function summary lines start with a third person singular present tense verb. Let's see if I got it right.

+++ b/core/modules/field/tests/src/Unit/Plugin/migrate/process/d6/FieldTypeDefaultsTest.php
@@ -0,0 +1,59 @@
+   * Test various default cases.

s/Test/Tests

+++ b/core/modules/field/tests/src/Unit/Plugin/migrate/process/d6/FieldTypeDefaultsTest.php
@@ -0,0 +1,59 @@
+   * When given an array that is not a date field we throw an exception.

Change to something like "Tests that an exception is thrown when the input is not a date field."

From IRC discussion with heddn, I've made a new patch to address items in #7.

And further from that discussion, IIRC, it was suggested that I could RTBC this after making the small change above.

Committed e145d63 and pushed to 8.0.x and 8.1.x. Thanks!