Meta: Build AK and LibRegex tests in Lagom and for Serenity

These tests were never built for the serenity target. Move their Lagom
build steps to the Lagom CMakeLists.txt, and add serenity build steps
for them. Also, fix the build errors when building them with the
serenity cross-compiler :^)

Userland/Libraries/LibRegex/Tests/CMakeLists.txt

@@ -4,17 +4,6 @@ file(GLOB REGEX_SOURCES CONFIGURE_DEPENDS "../*.cpp" "../C/*.cpp")
 foreach(source ${TEST_SOURCES})
     get_filename_component(name ${source} NAME_WE)
     add_executable(${name} ${source} ${REGEX_SOURCES})
-    target_link_libraries(${name} LagomCore)
-    add_test(
-        NAME ${name}
-        COMMAND ${name}
-        WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
-    )
-
-    set_tests_properties(
-        ${name}
-        PROPERTIES
-            FAIL_REGULAR_EXPRESSION
-            "FAIL"
-    )
+    target_link_libraries(${name} LibCore)
+    install(TARGETS ${name} RUNTIME DESTINATION usr/Tests/LibRegex)
 endforeach()