tests: Don't use the initdb template for older installations

Hi,

The attached patch fixes the issue that the initdb template mechanism is used
during ->init(), even install_path => ... was passed to Cluster->new().

Obviously using the template, created for the "current" tree, for an older
install won't normally work.

The only reason this hasn't caused breakage so far is that all the existing
tests that use an older node can't use the template for other reasons (because
they pass arguments to ->init()).

I nonticed this when writing a test that didn't pass anything to ->init()...

Planning to push this soon, don't think there's a reason to hold off.

Greetings,

Andres Freund