fix(testing/cmake/CMakeLists.txt): + libhipcxx_DIR

In addition to

```
-D "NVBench_DIR=${tmp_install_prefix}/lib/cmake/nvbench/"
```

we add

```
-D "libhipcxx_DIR=${tmp_install_prefix}/lib/rapids/cmake/libhipcxx/"
```

This fixes the last failing test `nvbench.test.cmake.test_export.install_tree` to 50 %.

Verbose test output:

    ```cmake
    36: Running test command: "/opt/conda/envs/cudf_dev/bin/ctest" "--output-on-failure"
    36: Test command failed: /opt/conda/envs/cudf_dev/bin/ctest
    36: Test project /home/mohammad/docharri/hipdf/DEPS/hipbench/build/testing/cmake/test_export/install_tree
    36:     Start 1: test_bench
    36: 1/2 Test #1: test_bench .......................   Passed    1.65 sec
    36:     Start 2: nvbench_ctl
    36: 2/2 Test #2: nvbench_ctl ......................***Failed    0.00 sec
    36: /home/mohammad/docharri/hipdf/DEPS/hipbench/build/testing/cmake/test_nvbench_install/bin/nvbench-ctl: error while loading shared libraries: libnvbench.so: cannot open shared object file: No such file or directory
    ```

The respective code is in:

`testing/cmake/test_export/CMakeLists.txt`

This file contains a section with the comment "# Setup LD_LIBRARY_PATH for testing".
Very likely this has something to do with the remaining issue.

Author: domcharrier

testing/cmake/CMakeLists.txt

@@ -58,6 +58,7 @@ nvbench_add_compile_test(test_name
   test_export
   build_tree
   -D "NVBench_DIR=${NVBench_BINARY_DIR}"
+  -D "libhipcxx_DIR=${libhipcxx_BINARY_DIR}"
   -D TEST_TYPE=BUILD_TREE
 )
 
@@ -69,6 +70,7 @@ nvbench_add_compile_test(test_name
   install_tree
   # "rapids_export() always installs to lib" per rapids_export docs
   -D "NVBench_DIR=${tmp_install_prefix}/lib/cmake/nvbench/"
+  -D "libhipcxx_DIR=${tmp_install_prefix}/lib/rapids/cmake/libhipcxx/"
   -D TEST_TYPE=INSTALL_TREE
 )
 set_tests_properties(${test_name} PROPERTIES FIXTURES_REQUIRED install_tree)