Add buildtests_gpu and check_gpu to simplify GPU testing.

This is in preparation of adding GPU tests to the CI, allowing
us to limit building/testing of GPU-specific tests for a given
GPU-capable runner.

GPU tests are tagged with the label "gpu".  The new targets
```
make buildtests_gpu
make check_gpu
```
allow building and running only the gpu tests.

cmake/EigenConfigureTesting.cmake

@@ -11,6 +11,15 @@ add_custom_target(buildtests)
 add_custom_target(check COMMAND "ctest")
 add_dependencies(check buildtests)
 
+# Convenience target for only building GPU tests.
+add_custom_target(buildtests_gpu)
+add_custom_target(check_gpu COMMAND "ctest" "--output-on-failure" 
+                                            "--no-compress-output"
+                                            "--build-no-clean"
+                                            "-T" "test"
+                                            "-L" "gpu")
+add_dependencies(check_gpu buildtests_gpu)
+
 # check whether /bin/bash exists (disabled as not used anymore)
 # find_file(EIGEN_BIN_BASH_EXISTS "/bin/bash" PATHS "/" NO_DEFAULT_PATH)