test: added googletest submodule and cmake files

.gitmodules

@@ -1,3 +1,6 @@
 [submodule "lib/SparkFun_ICM-20948_ParticleLibrary"]
 	path = lib/SparkFun_ICM-20948_ParticleLibrary
 	url = https://github.com/UCSD-E4E/SparkFun_ICM-20948_ParticleLibrary.git
+[submodule "external/googletest"]
+	path = external/googletest
+	url = https://github.com/google/googletest.git

gtests/CMakeLists.txt

@@ -0,0 +1,18 @@
+cmake_minimum_required(VERSION 3.14)
+project(TestScheduler)
+
+set(CMAKE_CXX_STANDARD 17)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+
+add_definitions(-DTEST_VERSION=1 -DGTESTING=1)
+include_directories($ENV{HOME}/local/include)
+link_directories($ENV{HOME}/local/lib)
+
+set(SOURCE_FILES
+    ../tests/scheduler_test_system.cpp
+    ../src/scheduler.cpp
+    ${CMAKE_CURRENT_SOURCE_DIR}/scheduler_functions.cpp
+    ${CMAKE_CURRENT_SOURCE_DIR}/gtest.cpp)
+
+add_executable(test_scheduler.o ${SOURCE_FILES})
+target_link_libraries(test_scheduler.o gtest gtest_main pthread)