Add unit test for aix memory plugin
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```ruby
#
# License:: Apache License, Version 2.0
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
#

require File.expand_path(File.dirname(__FILE__) + '/../../../spec_helper.rb')

describe Ohai::System, "AIX memory plugin" do
   before(:each) do
    @plugin = get_plugin("aix/memory")
    allow(@plugin).to receive(:collect_os).and_return(:aix)
    allow(@plugin).to receive(:shell_out).with("svmon -G -O unit=MB,summary=longreal | grep '[0-9]'").and_return(mock_shell_out(0, " 513280.00 340034.17 173245.83   62535.17 230400.05 276950.14  70176.00\n", nil))
  end

  it "should get total memory" do
    @plugin.run
    expect(@plugin['memory']['total']).to eql("513280.00")
  end
  
  it "should get free memory" do
    @plugin.run
    expect(@plugin['memory']['free']).to eql("173245.83")
  end
end 
```
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