[MacPorts] #16168: document test.env in the guide

[MacPorts]

#16168: document test.env in the guide
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 Reporter:  ryandesign at macports.org  |       Owner:  markd at macports.org     
     Type:  defect                   |      Status:  new                    
 Priority:  Normal                   |   Milestone:  Website & Documentation
Component:  guide                    |     Version:  1.6.0                  
 Keywords:                           |  
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 Document the existence and purpose of `test.env` in the Guide. For example
 some ports cannot be tested unless you set `DYLD_LIBRARY_PATH` to the path
 to where the libraries are within `${worksrcpath}` in the `test.env`.

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Ticket URL: <http://trac.macports.org/ticket/16168>
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