Fixed --keep-prefix functionality.

Changing to keep include prefixes had two side effects: the main
file being parsed wasn't filtered out anymore, and include directory
paths would be added to the path in the include statement.

Also moved the include_test*.fbs files to sub directories so we
can actually test the handling of -I etc.

tested: on Linux.

Change-Id: Ibae095cea7ab0cccbac15cfb5171719f6b5cad8c

CMakeLists.txt

@@ -195,6 +195,7 @@ function(compile_flatbuffers_schema_to_cpp SRC_FBS)
     OUTPUT ${GEN_HEADER}
     COMMAND "${FLATBUFFERS_FLATC_EXECUTABLE}" -c --no-includes --gen-mutable
             --gen-object-api -o "${SRC_FBS_DIR}"
+            -I "${CMAKE_CURRENT_SOURCE_DIR}/tests/include_test"
             "${CMAKE_CURRENT_SOURCE_DIR}/${SRC_FBS}"
     DEPENDS flatc)
 endfunction()