cmake: Enable googletest unit tests (#1720)

Also don't inherit compile warnings from wpiutil

There's not a good way to disable inherited compiler flags.

cmake/modules/AddTest.cmake

@@ -0,0 +1,14 @@
+include(CompileWarnings)
+
+macro(wpilib_add_test name srcdir)
+    file(GLOB_RECURSE test_src ${srcdir}/*.cpp)
+    add_executable(${name}_test ${test_src})
+    wpilib_target_warnings(${name}_test)
+    if (BUILD_SHARED_LIBS)
+        target_compile_definitions(${name}_test PRIVATE -DGTEST_LINKED_AS_SHARED_LIBRARY)
+    endif()
+    if (MSVC)
+        target_compile_options(${name}_test PRIVATE /wd4251 /wd4101)
+    endif()
+    add_test(NAME ${name} COMMAND ${name}_test)
+endmacro()