Build tests depending on installed files

When building and testing the 0.91-pre0 release in a clean chroot, I saw several test failures as below.
I believe that this is due to inkscape looking for the units.xml file in the system datadir while not being yet installed.
This should be reproducible with the usual `make; make check; sudo make install` on a machine where inkscape is NOT installed.
I'm unsure however about the correct approach to fix this, probably some hardcoded fallback value in units.cpp?

** (process:677): WARNING **: Units file /usr/share/inkscape/ui/units.xml is missing: Failed to open file '/usr/share/inkscape/ui/units.xml': No such file or directory

Running 107 tests.......................
In StyleTest::testOne:
./style-test.h:235: Error: Expected (std::string(str0_set) == std::string(cases[i].dst)), found ("font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;font-size:12px;line-height:14pt;font-family:sans-serif" != "font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;font-size:15px;line-height:14pt;font-family:sans-serif")
./style-test.h:235: Error: Expected (std::string(str0_set) == std::string(cases[i].dst)), found ("font-style:oblique;font-variant:normal;font-weight:normal;font-stretch:condensed;font-size:12px;line-height:normal;font-family:\'\"Helvetica Neue\", serif\'" != "font-style:oblique;font-variant:normal;font-weight:normal;font-stretch:condensed;font-size:15px;line-height:normal;font-family:\'\"Helvetica Neue\", serif\'")
./style-test.h:235: Error: Expected (std::string(str0_set) == std::string(cases[i].dst)), found ("font-size:12px" != "font-size:15px")
.....
In CurveTest::testGetSegmentCount:
./display/curve-test.h:56: Error: Expected (SPCurve(pv).get_segment_count() == 3u), found (2 != 3)
./display/curve-test.h:71: Error: Expected (curve.get_segment_count() == 10u), found (9 != 10)
In CurveTest::testNodesInPath:
./display/curve-test.h:108: Error: Expected (curve.nodes_in_path() == 12u), found (11 != 12)
...................
Failed 21 of 107 tests
Success rate: 80%
 0, segment: 3
XFAIL: cxxtests

The full test log is available at http://paste.debian.net/102242/