py_test legacy_create_init should be False by default

[shs96c]

Having the legacy_create_init attribute default to True is deeply confusing. Enabling legacy behaviour by default is a quirky thing to do, given that this seems to be an artefact of how Google use blaze. I believe that setting the default value to False will follow the principle of least surprise for most users.

[brandjon]

Indeed, it's something we want to clean up by changing the default and then removing the flag entirely (under the principle that we should avoid global flags). Closing as dup of bazelbuild/bazel#7386.