Add CMake install rule for tests (#483)

This PR adds a CMake `install` rule for test targets. This step is a prerequisite to being able to package tests in their own `conda` package, which will enable us to deprecate _Project Flash_.

Authors:
  - AJ Schmidt (https://github.com/ajschmidt8)
  - Corey J. Nolet (https://github.com/cjnolet)

Approvers:
  - Robert Maynard (https://github.com/robertmaynard)

URL: #483

cpp/test/CMakeLists.txt

@@ -115,6 +115,7 @@ PROPERTIES BUILD_RPATH                         "\$ORIGIN"
            CUDA_STANDARD_REQUIRED              ON
            POSITION_INDEPENDENT_CODE           ON
            INTERFACE_POSITION_INDEPENDENT_CODE ON
+           INSTALL_RPATH "\$ORIGIN/../../../lib"
 )
 
 target_compile_options(test_raft
@@ -140,3 +141,10 @@ PRIVATE
   $<TARGET_NAME_IF_EXISTS:OpenMP::OpenMP_CXX>
   $<TARGET_NAME_IF_EXISTS:conda_env>
 )
+
+install(
+    TARGETS test_raft
+    COMPONENT testing
+    DESTINATION bin/libraft/gtests
+    EXCLUDE_FROM_ALL
+)