8.3: How to create a node via REST with html?
Drupal is probably expecting an array of field data, even if the field itself only accepts a single value.

When the JSON is decoded, this is still an array:

{
  "format": "basic_html", 
  "value": "&lt;b&gt;This is the body of the test&lt;/b&gt;"
},


But with 2 items, when only one is expected.

You should be able to fix by providing an array with just the one item:

"body": [{
  "format": "basic_html", 
  "value": "&lt;b&gt;This is the body of the test&lt;/b&gt;"
}],