Convert tests to use plain rspec-puppet

Tests in the new style produces the following documentation output:

abs
  should not eq nil
  should run abs() and raise an Puppet::ParseError
  should run abs(-34) and return 34
  should run abs("-34") and return 34
  should run abs(34) and return 34
  should run abs("34") and return 34

1 files changed, 17 insertions, 7 deletions

diff --git a/spec/functions/type_of_spec.rb b/spec/functions/type_of_spec.rb
index b823460..f770990 100644
--- a/spec/functions/type_of_spec.rb
+++ b/spec/functions/type_of_spec.rb
@@ -1,13 +1,23 @@
-#! /usr/bin/env ruby -S rspec
-
 require 'spec_helper'
 
-describe 'type_of', :if => Puppet.version.to_f >= 4.0 do
-  it 'gives the type of a string' do
-    expect(subject.call_function('type_of', 'hello world')).to be_kind_of(Puppet::Pops::Types::PStringType)
+if ENV["FUTURE_PARSER"] == 'yes'
+  describe 'type_of' do
+    pending 'teach rspec-puppet to load future-only functions under 3.7.5'
   end
+end
+
+if Puppet.version.to_f >= 4.0
+  describe 'type_of' do
+    it { is_expected.not_to eq(nil) }
+    it { is_expected.to run.with_params().and_raise_error(ArgumentError) }
+    it { is_expected.to run.with_params('', '').and_raise_error(ArgumentError) }
+
+    it 'gives the type of a string' do
+      expect(subject.call({}, 'hello world')).to be_kind_of(Puppet::Pops::Types::PStringType)
+    end
 
-  it 'gives the type of an integer' do
-    expect(subject.call_function('type_of', 5)).to be_kind_of(Puppet::Pops::Types::PIntegerType)
+    it 'gives the type of an integer' do
+      expect(subject.call({}, 5)).to be_kind_of(Puppet::Pops::Types::PIntegerType)
+    end
   end
 end