Fix test suite when mysqld is not installed

The fact being confined, no value was returned when mysqld was not
found.  Also stub the which method to fix the problem.

Author: smortex

spec/unit/facter/mysqld_version_spec.rb

@@ -8,6 +8,7 @@
   describe 'mysqld_version' do
     context 'with value' do
       before :each do
+        Facter::Core::Execution.stubs(:which).with('mysqld').returns('/usr/sbin/mysqld')
         Facter::Util::Resolution.stubs(:exec).with('mysqld --no-defaults -V 2>/dev/null').returns('mysqld  Ver 5.5.49-37.9 for Linux on x86_64 (Percona Server (GPL), Release 37.9, Revision efa0073)')
       end
       it {