tests/java: force latency in generation to exercise race

Because we are racing here. In reality *all* of the java in that target
may rely on the generated file, so we need to block all of them, like we
would for headers in C/C++.

test cases/java/8 codegen custom target/com/mesonbuild/meson.build

@@ -4,5 +4,5 @@ config_file = custom_target('confgen',
                             input : 'Config.java.in',
                             output : 'Config.java',
                             command : [python, '-c',
-                              'import shutil; import sys; shutil.copy(sys.argv[1], sys.argv[2])',
+                              'import shutil, sys, time; time.sleep(1); shutil.copy(sys.argv[1], sys.argv[2])',
                               '@INPUT@', '@OUTPUT@'])