Test sketch that includes from subfolder

There was already a test for including from inside the `src/` subfolder,
but including from other subdirectories should also be supported (even
when source files inside those directories are not compiled). This adds
a test for this. See also arduino/Arduino#5186.

Signed-off-by: Matthijs Kooijman <matthijs@stdin.nl>

Author: matthijskooijman

src/arduino.cc/builder/test/sketch_with_subfolders/sketch_with_subfolders.ino

@@ -1,3 +1,4 @@
+#include "subfolder/other.h"
 #include "src/subfolder/other.h"
 
 MyClass myClass;

src/arduino.cc/builder/test/sketch_with_subfolders/subfolder/other.h

@@ -0,0 +1,2 @@
+// This file *is* included from the main sketch, even though it is
+// outside of the src/ directory.