Move cross-thread async tests into a separate file.

This commit does not change any of the code.

A subsequent commit will take advantage of this to apply the exact same tests when an EventPort is in use.

c++/Makefile.am

@@ -460,6 +460,7 @@ else !LITE_MODE
 check_PROGRAMS = capnp-test capnp-evolution-test capnp-afl-testcase
 heavy_tests =                                                  \
   src/kj/async-test.c++                                        \
+  src/kj/async-xthread-test.c++                                \
   src/kj/async-unix-test.c++                                   \
   src/kj/async-win32-test.c++                                  \
   src/kj/async-io-test.c++                                     \

c++/src/kj/CMakeLists.txt

@@ -215,6 +215,7 @@ if(BUILD_TESTING)
   if(NOT CAPNP_LITE)
     add_executable(kj-heavy-tests
       async-test.c++
+      async-xthread-test.c++
       async-unix-test.c++
       async-win32-test.c++
       async-io-test.c++