Tests: Update full post content tests to use snapshots

[gziolo]

This PR updates full post content tests to use snapshots. Diff is quite big, but changes were quite trivial:

• Readme file got updated, might require some editorial work
• I created 4 set of tests to keep assertions in their own methods
• 3 tests verify logic using snapshots
• I removed all no longer needed fixtures that aren't the initial post contents.

Testing

We need to verify if snapshots were properly created. From what I checked, they should be good.

Execute: npm run test-unit and make sure it still works.

Test Suites: 1 skipped, 31 passed, 31 of 32 total
Tests: 37 skipped, 571 passed, 608 total
Snapshots: 162 passed, 162 total
Time: 3.321s
Ran all test suites.

real 0m4.486s
user 0m22.232s
sys 0m1.611s

[gziolo]

It might be obsolete, but we can give it a try. Whenever we add/remove file, we would have to update this number.

[gziolo]

Again, this check would help us control how many block types are processed. If we don't want to keep it updated whenever block type is added/removed, we should at least verify if this number is greater than 0.

[gziolo]

It looks like phpunit tests depends on parsed fixtures:

There were 54 errors:

1. Parsing_Test::test_parser_output with data set #0 ('core-embed__animoto.html', 'core-embed__animoto.parsed.json')
   Exception: Missing fixture file: '/home/travis/build/WordPress/gutenberg/blocks/test/fixtures/core-embed__animoto.parsed.json'

I'm wondering if we can read them from snaphshot...

[nylen]

I am pretty disappointed with Jest snapshots for this particular task. They break most of the design goals of this test suite:

• Store fixture data in a way that is relatively easy to access and update (one clearly named set of files per test). Instead we end up with a single enormous snapshot file, which is all but guaranteed to be updated without close scrutiny of any changes.
• Automatically share fixtures between client and server. It is very important to ensure identical parser behavior on the server and the client, but we would have to write a lot of custom logic to re-use the Jest snapshots in PHP. I don't think it is worth it.
• Automatically verify that every registered block has at least one parsing/serialization test, with no updates required to the test code itself. Otherwise it is basically guaranteed that we will add blocks with zero tests.

The structure of this test suite (one file that processes all of these fixtures) is part of the problem. I'm fine with any solution or improvement here that decreases the complexity while preserving the above goals, but I don't think this is it. I'd rather see us using Jest snapshots for component testing as per their original purpose and as previously discussed at #1788.

[gziolo]

@nylen I must agree with you. If we didn't have to share all fixtures with PHP codebase this might work as expected, but given that Jest produces one snapshot file per test suite it doesn't make sense to promote usage of snapshots here.

Test file looks much cleaner with the changes applied in my opinion, so it might be a nice improvement if JS was our only concern here. Thanks for leaving your valuable feedback here. 👍

[nylen]

See #2018 for more discussion about using snapshots, this time for component testing.