Fix #380: Exclude unit tests from lcov report

[skliper]

Describe the contribution
Fixes #380, exclude unit-test files from lcov report

Testing performed
Steps taken to test the contribution:

1. Build/test/run lcov

make distclean
make ENABLE_UNIT_TESTS=TRUE SIMULATION=native prep
make
make install
make test
make lcov

1. Processed files no longer include unit test code, confirmed processed file list against repo
2. Coverage from 6.7.0 was

lines......: 92.6% (20233 of 21841 lines)
functions..: 95.9% (827 of 862 functions)

with unit test files included, now

  lines......: 98.6% (8195 of 8308 lines)
  functions..: 99.6% (471 of 473 functions)

Expected behavior changes
Valid coverage of flight code now reported

System(s) tested on:

• Hardware: cFS Dev Server
• OS: Ubuntu 16.04
• Versions: master bundle with this change

Additional context
None

Contributor Info
Jacob Hageman - NASA/GSFC