Remove order dependency from functions integration spec

The test in spec/integration/parser/functions_spec.rb would fail when
the spec tests were run in the order (among other orderings):

spec/unit/parser/functions/tag_spec.rb
spec/unit/parser/templatewrapper_spec.rb
spec/integration/parser/functions_spec.rb

There are tests that would cause the "template" function to be loaded
into the root environment.
Puppet::Parser::Functions.function("template") would then detect its
presence and P::P::F.rmfunction("template") would fail since #function(...)
looks in more than just the current environment to see if a function
is defined, while #rmfunction(...) only looks in the current
environment to see if a function can be removed.

In the test ordering specified earlier, tag_spec.rb would load the
"template" function, and templatewrapper_spec.rb would create a
current environment that would mask the root environment for #rmfunction(...),
but not for #function(...)

Since #rmfunction(...) only looks in the current environment, we
should be using #functions.include?("template") since that matches the
check that #rmfunction(...) itself uses.

Paired-with: Nick Lewis <nick@puppetlabs.com>