Update test/README.md

Author: sequba

.gitignore

@@ -22,3 +22,5 @@ node_modules/
 *.iml
 dev*.html
 .DS_Store
+
+/test/hyperformula-tests/

package.json

@@ -75,7 +75,7 @@
     "verify:cjs": "node script/check-file.js commonjs",
     "verify:publish-package": "npm pack | node script/check-publish-package.js",
     "verify:typings": "tsc --noEmit",
-    "test": "npm-run-all test:fetch-private lint test:jest test:browser",
+    "test": "npm-run-all lint test:jest test:browser",
     "test:fetch-private": "bash test/fetch-tests.sh",
     "test:jest": "cross-env NODE_ICU_DATA=node_modules/full-icu jest",
     "test:watch": "npm run test:jest -- --watch",

test/.gitignore

@@ -5,3 +5,54 @@ This folder contains only simple smoke tests that verify core HyperFormula funct
 HyperFormula team maintains a comprehensive test suite that includes unit tests run in different environments, compatibility tests, and performance tests.
 
 The full test suite is available on request. To obtain it, contact us at [email protected].
+
+## Using the private test suite
+
+The private test suite is kept in the `hyperformula-tests` repository. Once you have access to it, you can use it following this instructions:
+
+Whenenever you switch branch in the main repository, you need to fetch the private test suite to the `test/hyperformula-tests` directory by running:
+```
+npm run test:fetch-private
+
+```
+
+Then, you can run the test by calling one og the commands:
+
+```
+npm run test
+npm run test:jest
+npm run test:browser
+npm run test:coverage
+npm run test:performance
+npm run test:compatibility
+```
+
+## `fetch-tests.sh`
+
+This file is located in the `test` directory.
+
+### What it does
+
+1. **Clone if missing** – If `test/hyperformula-tests` does not exist:
+   - With `DEPLOY_TOKEN` set: clones via HTTPS using the token
+   - Otherwise: clones via SSH (`[email protected]:handsontable/hyperformula-tests.git`)
+
+2. **Detect current branch** – Uses:
+   - `GITHUB_HEAD_REF` (pull request source branch)
+   - `GITHUB_REF_NAME` (push branch)
+   - `git rev-parse --abbrev-ref HEAD` (local), falling back to `develop` on detached HEAD
+
+3. **Checkout matching branch** – In `hyperformula-tests`:
+   - Fetches from `origin`
+   - Checks out the branch if it exists locally or on `origin`
+   - Otherwise checks out `develop`
+
+4. **Pull latest** – Runs `git pull origin` on the checked-out branch.
+
+### Environment variables
+
+| Variable        | Purpose                                                                 |
+|-----------------|-------------------------------------------------------------------------|
+| `DEPLOY_TOKEN`  | GitHub token for HTTPS clone (used in CI when SSH is not available)     |
+| `GITHUB_HEAD_REF` | Source branch for pull requests (set by GitHub Actions)             |
+| `GITHUB_REF_NAME` | Branch/tag that triggered the workflow (set by GitHub Actions)       |