✅ 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>

ini/native.ini

@@ -15,13 +15,24 @@
 [env:linux_native]
 platform         = native
 framework        =
-build_flags      = -D__PLAT_LINUX__ -std=gnu++17 -ggdb -g -lrt -lpthread -D__MARLIN_FIRMWARE__ -Wno-expansion-to-defined
+build_flags      = ${common.build_flags} -D__PLAT_LINUX__ -std=gnu++17 -ggdb -g -lrt -lpthread -D__MARLIN_FIRMWARE__ -Wno-expansion-to-defined
 build_src_flags  = -Wall -IMarlin/src/HAL/LINUX/include
 build_unflags    = -Wall
 lib_ldf_mode     = off
-lib_deps         =
 build_src_filter = ${common.default_src_filter} +<src/HAL/LINUX>
 
+# Environment specifically for unit testing through the Makefile
+# This is somewhat unorthodox, in that it uses the PlatformIO Unity testing framework,
+# but actual targets are dynamically generated during the build. This seems to prevent
+# Unity from being automatically included, so it is added here.
+[env:linux_native_test]
+extends          = env:linux_native
+extra_scripts    = ${common.extra_scripts}
+                   post:buildroot/share/PlatformIO/scripts/collect-code-tests.py
+build_src_filter = ${env:linux_native.build_src_filter} +<tests>
+lib_deps         = throwtheswitch/Unity@^2.5.2
+test_build_src   = true
+
 #
 # Native Simulation
 # Builds with a small subset of available features