Move rtc_task_queue_for_test outside of the rtc_include_tests scope.

I hit a problem in a separate CL where targets depended on
rtc_task_queue_for_test were being built while rtc_include_tests
was set to false. So this addresses a future problem.

Bug: webrtc:8848
Change-Id: Id049049d60edd6abdb6d9c56162b7554dc48b057
Reviewed-on: https://webrtc-review.googlesource.com/54880
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Tommi <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22078}

rtc_base/BUILD.gn

@@ -888,6 +888,21 @@ rtc_source_set("rtc_base_tests_utils") {
   ]
 }
 
+rtc_source_set("rtc_task_queue_for_test") {
+  visibility = [ "*" ]
+  testonly = true
+
+  sources = [
+    "task_queue_for_test.cc",
+    "task_queue_for_test.h",
+  ]
+  deps = [
+    ":checks",
+    ":rtc_base_approved",
+    ":rtc_task_queue",
+  ]
+}
+
 if (rtc_include_tests) {
   rtc_source_set("rtc_base_tests_main") {
     testonly = true
@@ -997,21 +1012,6 @@ if (rtc_include_tests) {
     ]
   }
 
-  rtc_source_set("rtc_task_queue_for_test") {
-    visibility = [ "*" ]
-    testonly = true
-
-    sources = [
-      "task_queue_for_test.cc",
-      "task_queue_for_test.h",
-    ]
-    deps = [
-      ":checks",
-      ":rtc_base_approved",
-      ":rtc_task_queue",
-    ]
-  }
-
   rtc_source_set("rtc_task_queue_unittests") {
     visibility = [ "*" ]
     testonly = true