Revert "tests: Use parent module includes when used as a submodule"

This reverts commit 120d571e8e835afde4a5c31fdc26c2452c0b54d7.

Conflicts:
* tests/CMakeLists.txt:
  * target_include_directories: the instruction whose addition is to
    be reverted has changed. Remove what is there now.
diff --git a/tests/Makefile b/tests/Makefile
index 15564c5..e74bf95 100644
--- a/tests/Makefile
+++ b/tests/Makefile
@@ -6,8 +6,7 @@
 WARNING_CFLAGS ?= -Wall -Wextra
 LDFLAGS ?=
 
-CRYPTO_INCLUDES ?= -I../include
-LOCAL_CFLAGS = $(WARNING_CFLAGS) $(CRYPTO_INCLUDES) -I../library -D_FILE_OFFSET_BITS=64
+LOCAL_CFLAGS = $(WARNING_CFLAGS) -I../include -I../library -D_FILE_OFFSET_BITS=64
 LOCAL_LDFLAGS = -L../library			\
 		-lmbedtls$(SHARED_SUFFIX)	\
 		-lmbedx509$(SHARED_SUFFIX)	\