fix test dependencies

[nim65s]

Hi,

Running tests with CTEST_PARALLEL_LEVEL > 1, I get the following error:

3/14 Test  #5: ReadGraph ........................***Failed    0.00 sec
ERROR: Filestream to test.graph not open, nothing read.
test failed (EXPECT_TRUE) in octomap/src/testing/unit_tests.cpp, line 154

So I guess we need to explain to CMake that ReadGraph must wait for InsertScan, ref:

octomap/octomap/src/testing/unit_tests.cpp

Line 147 in b7118e0

EXPECT_TRUE (graph->writeBinary("test.graph"));

[ahornung]

Indeed, the tests are not really independent of each other. Thanks for the fix!