Fix installing extensions with --install-dir

When the extension building code was moved from Gem::Installer to
Gem::Ext::Builder the dependency on gem_dir was not correctly migrated.

This meant that the gem_dir computed in Gem::Ext::Builder was incorrect
so RubyGems would attempt to install the extension in GEM_HOME not the
user's chosen directory.

Issue #642
Switch assert File.exist? to assert_path_exists

There were multiple styles of this assertion depending upon the age of
the test.  Now we consistently use assert_path_exists.