✅ Add unit testing framework (#26948)

- Add a framework to build and execute unit tests for Marlin.
- Enable unit test execution as part of PR checks.

---------

Co-authored-by: Costas Basdekis <costas.basdekis@gmail.com>
Co-authored-by: Scott Lahteine <thinkyhead@users.noreply.github.com>

.github/workflows/ci-build-tests.yml

@@ -1,9 +1,9 @@
 #
-# test-builds.yml
+# ci-build-tests.yml
 # Do test builds to catch compile errors
 #
 
-name: CI
+name: CI - Build Tests
 
 on:
   pull_request:
@@ -27,7 +27,7 @@ on:
 
 jobs:
   test_builds:
-    name: Run All Tests
+    name: Build Test
     if: github.repository == 'MarlinFirmware/Marlin'
 
     runs-on: ubuntu-latest

.github/workflows/ci-unit-tests.yml

@@ -0,0 +1,73 @@
+#
+# ci-unit-tests.yml
+# Build and execute unit tests to catch functional issues in code
+#
+
+name: CI - Unit Tests
+
+on:
+  pull_request:
+    branches:
+    - bugfix-2.1.x
+    # Cannot be enabled on 2.1.x until it contains the unit test framework
+    #- 2.1.x
+    paths-ignore:
+    - config/**
+    - data/**
+    - docs/**
+    - '**/*.md'
+  push:
+    branches:
+    - bugfix-2.1.x
+    # Cannot be enabled on 2.1.x until it contains the unit test framework
+    #- 2.1.x
+    paths-ignore:
+    - config/**
+    - data/**
+    - docs/**
+    - '**/*.md'
+
+jobs:
+  # This runs all unit tests as a single job. While it should be possible to break this up into
+  # multiple jobs, they currently run quickly and finish long before the compilation tests.
+  run_unit_tests:
+    name: Unit Test
+    # These tests will only be able to run on the bugfix-2.1.x branch, until the next release
+    # pulls them into additional branches.
+    if: github.repository == 'MarlinFirmware/Marlin'
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - name: Check out the PR
+      uses: actions/checkout@v4
+
+    - name: Cache pip
+      uses: actions/cache@v4
+      with:
+        path: ~/.cache/pip
+        key: ${{ runner.os }}-pip-${{ hashFiles('**/requirements.txt') }}
+        restore-keys: |
+          ${{ runner.os }}-pip-
+
+    - name: Cache PlatformIO
+      uses: actions/cache@v4
+      with:
+        path: ~/.platformio
+        key: ${{ runner.os }}-${{ hashFiles('**/lockfiles') }}
+
+    - name: Select Python 3.9
+      uses: actions/setup-python@v5
+      with:
+        python-version: '3.9'
+        architecture: 'x64'
+
+    - name: Install PlatformIO
+      run: |
+        pip install -U platformio
+        pio upgrade --dev
+        pio pkg update --global
+
+    - name: Run All Unit Tests
+      run: |
+        make unit-test-all-local