gmxapi tests should not depend on command line tools
gmxapi tests use
grompp to prepare test simulation input since equivalent functionality is not available in the public API in GROMACS 2019.
This means that the gmxapi tests cannot be configured when cross-compiling (see #2669), so a fatal error is issued when both GMXAPI and BUILD_TESTING are ON if the CMAKE_CROSSCOMPILING variable is set (such as due to explicitly setting CMAKE_SYSTEM_NAME).
This issue could be resolved using existing testing framework utilities, as in https://gerrit.gromacs.org/c/8483/ , or by migrating to API features for simulation preparation, as they become available.
In either case, when gmxapi tests no longer rely on
grompp, the fatal error issued by CMake should be removed.