Improve sample_restraint project integration.

Improve sample_restraint project integration.

Build for testing as part of a gmxapi enabled GROMACS build. Fix some
errors and collisions.

* Use distinct CMake options for using bundled pybind sources in
  gmxapi Python package and sample_restraint package.
* Distinguish between "umbrella" build and standalone build when
  trying to determine googletest source for sample_restraint.
* Avoid a couple of warnings about unused variables.
* Use GTest::Main target more consistently for sample_restraint tests.

Refs #2056
Refs #3027
Refs #3033

Change-Id: I55d20e6228779ecc726587231c83298356192f6f