Test single exercise

[TheRealOwenRees]

• add script test:only which can accept -- filename e.g. npm run test:only -- tmp/tests/Acronym_test.res.js
• add a Makefile command to allow a single test to be ran (in preparation for single test instructions in docs), e.g. make test-one EXERCISE=eliuds-eggs
• re-sync package.json files
• update where the test files are ran - tmp/tests/*.js, which fixes the error below:

npm run ci

> ci
> npm test


> test
> rescript && retest tests/*.js

[1/3] 🧹 Cleaned 0/121 in 0.01s
[2/3] 🧱 Parsed 56 source files in 0.17s
[3/3] 🤺 Compiled 56 modules in 0.25s

✨ Finished Compilation in 0.44s
node:internal/modules/esm/resolve:274
    throw new ERR_MODULE_NOT_FOUND(
          ^

Error [ERR_MODULE_NOT_FOUND]: Cannot find module '/home/owen/Desktop/rescript/node_modules/rescript-test/src/Test.js' imported from /home/owen/Desktop/rescript/node_modules/rescript-test/bin/retest.mjs
    at finalizeResolution (node:internal/modules/esm/resolve:274:11)
    at moduleResolve (node:internal/modules/esm/resolve:864:10)
    at defaultResolve (node:internal/modules/esm/resolve:990:11)
    at #cachedDefaultResolve (node:internal/modules/esm/loader:718:20)
    at #resolveAndMaybeBlockOnLoaderThread (node:internal/modules/esm/loader:735:38)
    at ModuleLoader.resolveSync (node:internal/modules/esm/loader:764:52)
    at #resolve (node:internal/modules/esm/loader:700:17)
    at ModuleLoader.getOrCreateModuleJob (node:internal/modules/esm/loader:620:35)
    at onImport.tracePromise.__proto__ (node:internal/modules/esm/loader:649:32)
    at TracingChannel.tracePromise (node:diagnostics_channel:350:14) {
  code: 'ERR_MODULE_NOT_FOUND',
  url: 'file:///home/owen/Desktop/rescript/node_modules/rescript-test/src/Test.js'
}