nacl_loader_unittests should be a test target.

Tested with the following command lines on Linux:

$ gn gen out-gn --args=enable_nacl=true
$ ninja -C out-gn nacl_loader_unittests

Note: There are still many link errors with this target. They will be
addressed in followup patches.

BUG=432959
TEST=see above
R=bradn@google.com,dpranke@chromium.org
TBR=bradnelson@chromium.org

Review URL: https://codereview.chromium.org/1034723002

Cr-Commit-Position: refs/heads/master@{#322365}

components/nacl/BUILD.gn

@@ -4,6 +4,7 @@
 
 import("//build/config/features.gni")
 import("//build/config/ui.gni")
+import("//testing/test.gni")
 
 if (enable_nacl) {
   source_set("nacl") {
@@ -126,8 +127,7 @@ if (enable_nacl) {
     ]
   }
 
-  executable("nacl_loader_unittests") {
-    testonly = true
+  test("nacl_loader_unittests") {
     sources = [
       "loader/run_all_unittests.cc",
     ]