[libc] Actually run integration tests.

[sivachandra]

Summary

After the switch to add_custom_target to run integration tests, most
of them were not actually being run because of the difference in the way
the COMMAND value is treated between add_custom_target and
add_custom_command. This patch gets the integration tests to run
again by passing the correct set of arguments to add_custom_target.

[michaelrj]

LGTM with nit

libc/cmake/modules/LLVMLibCTestRules.cmake

537

nit: remove "to"

[sivachandra]

Address comment.

libc/cmake/modules/LLVMLibCTestRules.cmake

add_dependencies(${fq_build_target_name}
                    ${INTEGRATION_TEST_DEPENDS})
 
   # Tests on the GPU require an external loader utility to launch the kernel.
   if(TARGET libc.utils.gpu.loader)
     add_dependencies(${fq_build_target_name} libc.utils.gpu.loader)
     get_target_property(gpu_loader_exe libc.utils.gpu.loader "EXECUTABLE")
   endif()
 
-  add_custom_target(
-    ${fq_target_name}
-    COMMAND ${INTEGRATION_TEST_ENV}
-            $<$<BOOL:${LIBC_TARGET_ARCHITECTURE_IS_GPU}>:${gpu_loader_exe}>
-            ${INTEGRATION_TEST_LOADER_ARGS}
-            $<TARGET_FILE:${fq_build_target_name}> ${INTEGRATION_TEST_ARGS}
+  # We have to use a separate var to store the command as a list because
+  # the COMMAND option of `add_custom_target` cannot handle empty vars in the
+  # command. For example, if INTEGRATION_TEST_ENV is empty, the actual
+  # command also will not run. So, we use this list and tell `add_custom_target`
+  # to expand the list (by including the option COMMAND_EXPAND_LISTS). This
+  # makes `add_custom_target` construct the correct command and execute it.
+  set(test_cmd
+      ${INTEGRATION_TEST_ENV}
+      $<TARGET_FILE:${fq_build_target_name}> ${INTEGRATION_TEST_ARGS})
+  add_custom_target(
+    ${fq_target_name}
+    COMMAND ${test_cmd}
+    COMMAND_EXPAND_LISTS
     COMMENT "Running integration test ${fq_target_name}"
   )
   add_dependencies(${INTEGRATION_TEST_SUITE} ${fq_target_name})
 endfunction(add_integration_test)