ci: use esp8266 mock framework

Resolve the issue with the UnixHostDuino not really being compatible
with the esp8266 Core String (...and the rest of the Core, as well)

Port the CMakeLists.txt from the rpnlib and update it use FetchContent
instead of either manually fetching dependencies or using PIO artifacts
Caching is *expected* to work, but might need slight adjustments

.github/workflows/push.yml

@@ -17,16 +17,11 @@ jobs:
     - uses: actions/setup-python@v2
       with:
         python-version: '3.x'
-    - name: Cache pip
+    - name: Cache CMake
       uses: actions/cache@v2
       with:
-        path: ~/.cache/pip
-        key: ${{ runner.os }}-pip-${{ hashFiles('ci_install.sh') }}
-    - name: Cache PlatformIO
-      uses: actions/cache@v2
-      with:
-        path: ~/.platformio
-        key: ${{ runner.os }}-${{ hashFiles('code/platformio.ini') }}
+        path: code/test/unit/cache
+        key: ${{ runner.os }}-${{ hashFiles('code/test/unit/CMakeLists.txt') }}
     - name: Host tests
       run: |
         ./ci_install.sh host