CMake: rename test target to "tests"

Comply with CMake policy CMP0037: "test" is a reserved target name.

CMakeLists.txt

@@ -233,6 +233,6 @@ add_custom_target(static DEPENDS ${OWLPS_STATIC_TARGETS})
 
 ### Tests ###
 
-add_custom_target(test
+add_custom_target(tests
   COMMAND ${CMAKE_RUNTIME_OUTPUT_DIRECTORY}/test_positioner
   DEPENDS test_positioner)

INSTALL.t2t

@@ -148,9 +148,10 @@ To display all the available targets, type:
 
 = Running the tests =
 
-To run the unit tests, use the ``test`` target:
+To run the unit tests, you will need to install ``cxxtest``. Then use
+the ``tests`` target from your build directory:
 
-``` make test
+``` make tests
 
 Currently, only OwlPS Positioner has (partial) unit tests.