Too many CMake warnings about `dot`

[pcanal]

Running CMake for Phlex without dot lead to many warning like:

CMake Warning at test/CMakeLists.txt:10 (message):
  dot program not found; all TEST_DOT_GRAPH options will be ignored.
Call Stack (most recent call first):
  test/CMakeLists.txt:55 (dot_test)
  test/CMakeLists.txt:64 (add_catch_test)

independently from whether dot is needed or whether the warning is useless, we really only need the warning once.

[knoepfel]

Resolved by #30.