fix test jamfile referring to BOOST_ROOT instead of relative paths

[jeking3]

Referring to BOOST_ROOT for dependencies is bad; if "b2 headers" had not been run yet at the top level and you tried to build uuid separately, the test jamfile was wrong. With this fix it can build independently and seed the appropriate headers without a full build.

Without:

Performing configuration checks

    - default address-model    : 64-bit [1]
    - default architecture     : x86 [1]
    - symlinks supported       : yes
error: Unable to find file or target named
error:     '/boost/boost/uuid/uuid.hpp'
error: referred to from project at
error:     'test'

[codecov[bot]]

Codecov Report

Merging #130 (1462b83) into develop (0ee3b2b) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #130   +/-   ##
========================================
  Coverage    60.44%   60.44%           
========================================
  Files           34       34           
  Lines         1388     1388           
  Branches       593      593           
========================================
  Hits           839      839           
  Misses          55       55           
  Partials       494      494           

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