CMake build: pw_add_test updates; fix C builds
CMake build: pw_add_test updates; fix C builds

- In pw_add_test, require the name argument and accept a source list.
  This is more consistent with GN and Bazel.
- Make some compile options C++-only.

Change-Id: I5c11bcf5fb35c11fe6c332034c95ca6ad9203429

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed