Update googletest to use 1.17.0 exactly

With the '^x.y.z' semantics, the version can upgrade out from under us and change dependencies, so this pins to an exact version.

Author: mtlynch

platformio.ini

@@ -156,12 +156,12 @@ lib_deps =
 
 [env:native]
 platform = native
-build_flags = -std=c++14
+build_flags = -std=c++17
   -I src
   -I test/mocks
 test_build_src = yes
 build_src_filter =
   -<*>
   +<../src/Utils.cpp>
 lib_deps =
-  google/googletest @ ^1.15.2
+  google/googletest @ 1.17.0