(PDOC-3) Use constants instead of strings in tests

Prior to this commit, several rspec tests were using `decribe "string"'
instead of `describe constant`. Update the tests to use the constant instead.
Additionally, rename the puppet function tests to reflect the renaming of the
handlers.

spec/unit/puppetx/puppetlabs/strings/yard/defined_type_handler_spec.rb

@@ -3,7 +3,7 @@ require 'puppetx/puppetlabs/strings/yard/handlers/defined_type_handler'
 require 'strings_spec/parsing'
 
 
-describe "DefinedTypeHanlder" do
+describe Puppetx::PuppetLabs::Strings::YARD::Handlers::DefinedTypeHandler do
   include StringsSpec::Parsing
 
   def the_definedtype()

spec/unit/puppetx/puppetlabs/strings/yard/host_class_handler_spec.rb

@@ -2,7 +2,7 @@ require 'spec_helper'
 require 'puppetx/puppetlabs/strings/yard/handlers/host_class_handler'
 require 'strings_spec/parsing'
 
-describe "HostClassDefintion" do
+describe Puppetx::PuppetLabs::Strings::YARD::Handlers::HostClassHandler do
   include StringsSpec::Parsing
 
   def the_hostclass()

spec/unit/puppetx/puppetlabs/strings/yard/puppet_3x_function_handler_spec.rb

@@ -2,7 +2,7 @@ require 'spec_helper'
 require 'puppetx/puppetlabs/strings/yard/handlers/puppet_3x_function_handler'
 require 'strings_spec/parsing'
 
-describe "Puppet3xFunctionHanlder" do
+describe Puppetx::PuppetLabs::Strings::YARD::Handlers::Puppet3xFunctionHandler do
   include StringsSpec::Parsing
 
   def the_method()

spec/unit/puppetx/puppetlabs/strings/yard/puppet_4x_function_handler_spec.rb

@@ -2,7 +2,7 @@ require 'spec_helper'
 require 'puppetx/puppetlabs/strings/yard/handlers/puppet_4x_function_handler'
 require 'strings_spec/parsing'
 
-describe "Pupet4xFunctionHandler" do
+describe Puppetx::PuppetLabs::Strings::YARD::Handlers::Puppet4xFunctionHandler do
   include StringsSpec::Parsing
 
   def the_method()