[LibOS] Remove the reference of level variable from Makefile.Test

Remove the reference of level variable from Makefile.Test.
After this patch, level variable in Makefile's of tests can be removed.

LibOS/shim/test/Makefile.Test

@@ -1,9 +1,11 @@
+SHIM_TEST_DIR := $(dir $(lastword $(MAKEFILE_LIST)))
+
 SYS ?= $(shell gcc -dumpmachine)
 export SYS
 
-PALDIR  = $(level)../../../Pal/src
-SHIMDIR = $(level)../src
-RUNTIME = $(level)../../../Runtime
+PALDIR  = $(SHIM_TEST_DIR)/../../../Pal/src
+SHIMDIR = $(SHIM_TEST_DIR)/../src
+RUNTIME = $(SHIM_TEST_DIR)/../../../Runtime
 
 ifeq ($(SGX),1)
 PAL_LOADER = pal_loader SGX