Add readme file describing new tests

blocks/test/fixtures/README.md

@@ -0,0 +1,46 @@
+## Full post content test fixtures
+
+This directory contains sets of fixture files that are used to test the parsing
+and serialization logic.
+
+Each test is made up of three fixture files:
+
+1. `fixture-name.html`: The initial post content.
+2. `fixture-name.json`: The **expected** parsed representation of the block(s)
+   inside the post content, along with their attributes and any nested content.
+   The contents of this file are compared against the **actual** parsed block
+   object(s).
+3. `fixture-name.serialized.html`: The **expected** result of calling
+   `serialize` on the parsed block object(s).  The contents of this file are
+   compared against the **actual** re-serialized post content.  This final step
+   simulates opening and re-saving a post.
+
+Every block is required to have at least one such set of fixture files to test
+the parsing and serialization of that block.  These fixtures must be named like
+`core-blockname{-*,}.{html,json,serialized.html}`.  For example, for the
+`core/image` block, the following three fixture files must exist:
+
+1. `core-image.html` (or `core-image-specific-test-name.html`).  Must contain a
+   `<!-- wp:core/image -->` block.
+2. `core-image.json` (or `core-image-specific-test-name.html`).
+3. `core-image.serialized.html` (or
+   `core-image-specific-test-name.serialized.html`).
+
+Ideally all important attributes and features of the block should be tested
+this way.  New contributions in the form of additional test cases are always
+welcome - this is a great way for us to identify bugs and prevent them from
+recurring in the future.
+
+When adding a new test, it's only necessary to create file (1) above, then
+there is a command you can run to generate (2) and (3):
+
+```sh
+GENERATE_MISSING_FIXTURES=y npm run test-unit -- --grep 'full post content fixture'
+```
+
+However, when using this command, please be sure to manually verify that the
+contents of the `.json` and `.serialized.html` files are as expected.
+
+See the
+[`full-content.js`](../full-content.js)
+test file for the code that runs these tests.

blocks/test/full-content.js

@@ -26,6 +26,7 @@ const fixturesDir = path.join( __dirname, 'fixtures' );
 // Get the "base" name for each fixture first.
 const fileBasenames = uniq(
 	fs.readdirSync( fixturesDir )
+		.filter( f => /(\.html|\.json)$/.test( f ) )
 		.map( f => f.replace( /\..+$/, '' ) )
 );