LibJS: Skip test262 tests with the CanBlockIsFalse flag

From test262 documentation, this flag means:

    The test file should only be run when the [[CanBlock]] property of
    the Agent Record executing the file is `false`.

This patch stubs out the accessor for that internal slot and skips tests
with the CanBlockIsFalse if that internal slot is true.

Userland/Libraries/LibJS/CMakeLists.txt

@@ -35,6 +35,7 @@ set(SOURCES
     Print.cpp
     Runtime/AbstractOperations.cpp
     Runtime/Accessor.cpp
+    Runtime/Agent.cpp
     Runtime/AggregateError.cpp
     Runtime/AggregateErrorConstructor.cpp
     Runtime/AggregateErrorPrototype.cpp