ci(tests): add test framework (#1603)

tests/unit/test_file_handler.cpp

@@ -0,0 +1,19 @@
+/**
+ * @file tests/test_file_handler.cpp
+ * @brief Test src/file_handler.*.
+ */
+#include <src/file_handler.h>
+
+#include <tests/conftest.cpp>
+
+TEST(FileHandlerTests, WriteFileTest) {
+  EXPECT_EQ(file_handler::write_file("write_file_test.txt", "test"), 0);
+}
+
+TEST(FileHandlerTests, ReadFileTest) {
+  // read file from WriteFileTest
+  EXPECT_EQ(file_handler::read_file("write_file_test.txt"), "test\n");  // sunshine adds a newline
+
+  // read missing file
+  EXPECT_EQ(file_handler::read_file("non-existing-file.txt"), "");
+}