Remove duplicate lines from test CMakeLists

It appears that these four lines are exact copies of the four lines
above. Here's the values in PDAL_UNITTEST_SOURCES for my build:
https://gist.github.com/gadomski/5918454. The files in lines 1-51 of the
gist are duplicated in lines 52-102.
Merge pull request #151 from gadomski/duplicate-lines-in-test-cmake

Remove duplicate lines from test CMakeLists