Test blocks don't load on develop

[BeksOmega]

Describe the bug

Test blocks are no longer available in the playground on develop.

To Reproduce

Steps to reproduce the behavior:

1. Go to the playground.
2. Select the test blocks category.
3. Get the 'you need to run npm install' message, even if you have.

Expected behavior

Test blocks are available for testing.

Additional context

I've tried:

• Adding <script type="module" src="../node_modules/@blockly/block-test/dist/index.js"></script> to the playground below the main module.
• Adding a document.write call in at the end of the uncompressed part of prepare.js with <script src="../node_modules/@blockly/block-test/dist/index.js"></script>
• Adding import '../node_modules/@blockly/block-test/dist/index.js' to the playground below import Blockly from './prepare.js'
• Adding import '@blockly/block-test' to the playground below import Blockly from './prepare.js'
• Adding import '../node_modules/@blockly/block-test/dist/index.js' to blockly.mjs right before the default export.
• Adding import '@blockly/block-test' to blockly.mjs right before the default export.

For the full paths this gives the error:

Uncaught TypeError: Cannot read properties of undefined (reading 'Blockly')
    at universalModuleDefinition:7:98
    at universalModuleDefinition:1:1

And for the non-full paths this gives the error:

Uncaught TypeError: Failed to resolve module specifier "@blockly/block-test". Relative references must start with either "/", "./", or "../"

Related to #5931

[BeksOmega]

Closed by #6015