Build tests as a separate target

It is very annoying having to build all the tests when you are
testing some code in the main library. Someone downloading the
library probably doesn't need the tests either, so these are left
out of the default target.

CMakeLists.txt

@@ -217,9 +217,11 @@ cond_disable_test ("AGMG")
 cond_disable_test ("ERT")
 
 # compile each of these separately
+add_custom_target (tests)
 foreach (test_FILE IN LISTS tests_SOURCES)
 	get_filename_component (test_NAME "${test_FILE}" NAME_WE)
-	add_executable (${test_NAME} ${test_FILE})
+	add_executable (${test_NAME} EXCLUDE_FROM_ALL ${test_FILE})
+	add_dependencies (tests ${test_NAME})
 	set_target_properties (${test_NAME} PROPERTIES
 		LINK_FLAGS "${opm-core_LINKER_FLAGS_STR}"
 		)