Don't run post_install_test by default

[zjohl]

I'm attempting to upgrade to ruby 4 in one of my apps that uses dhash-vips. I ended up seeing an error from this gem:

Gem::Specification#activate_dependencies': Could not find 'ffi' (~> 1.12) among
339 total gem(s) (Gem::MissingSpecError)

...

make: *** [Makefile:3: test] Error 1
make failed, exit code 2

...

An error occurred while installing dhash-vips (0.2.4.0), and Bundler cannot
continue.
In Gemfile:
  dhash-vips

extconf.rb prepends a test:all target to the Makefile, which makes test the default make target. This was causing issues in the parallel install of our dependency image because ffi wasn't resolvable when dhash-vips built, so make test failed and aborted the install.

My fix for this was switching to append this target to the makefile, and renaming it, so it's not the default and won't be run on bundle install. I also updated the README to use make post_install_test instead of just make