Make KernelSystemCall more specific

Test that calls to`system` on modules other than `Kernel` are excluded,
such as in this example:

    module Foo
      def self.system(*args); end
    end

    # This is not a call to Kernel.system
    Foo.system("bar")

ql/test/library-tests/frameworks/CommandExecution.rb

@@ -66,4 +66,23 @@ Open3.pipeline("echo foo")
 
 <<`EOF`
 echo foo
-EOF
+EOF
+
+module MockSystem
+  def system(*args)
+    args
+  end
+
+  def self.system(*args)
+    args
+  end
+end
+
+class Foo
+  include MockSystem
+
+  def run
+    system("ls")
+    MockSystem.system("ls")
+  end
+end