Add the plugins dir to includes for testcases

This makes it possible for a testcase to include source files from plugin
directories. When the plugins got pulled out of the main Kaleidoscope source
dir, plugin header files became unavailable to testcases, but these files are
useful in some instances.

Signed-off-by: Michael Richters <gedankenexperimenter@gmail.com>

testing/makefiles/testcase.mk

@@ -3,6 +3,8 @@ top_dir         := $(abspath $(mkfile_dir)../..)
 
 include $(mkfile_dir)/shared.mk
 
+include_plugins_dir := -I${top_dir}/plugins \
+
 build_dir := ${top_dir}/_build/${testcase}
 
 LIB_DIR := ${build_dir}/lib
@@ -81,7 +83,7 @@ endif
 ${OBJ_DIR}/%.o: ${SRC_DIR}/%.cpp
 	$(QUIET) install -d "${OBJ_DIR}"
 	$(QUIET) $(COMPILER_WRAPPER) $(call _arduino_prop,compiler.cpp.cmd) -o "$@" -c -std=c++14 \
-		${shared_includes} ${shared_defines} $<
+		${shared_includes} ${include_plugins_dir} ${shared_defines} $<
 
 clean:
 	$(QUIET) rm -f -- "${SRC_DIR}/generated-testcase.cpp"