Use relative paths in Greentea test_spec.json

We had a similar change in #89 but that was not enough. This commit
ensures all paths in `test_spec.json` are actually relative.

Author: LDong-Arm

test_psa_target.py

@@ -248,9 +248,9 @@ def _get_test_spec(args, suite, binary_name):
     log_path = join("test", "logs", target, "{}.log".format(suite))
 
     if args.cli == 1:
-        image_path = join(ROOT, "BUILD", target, toolchain, binary_name)
+        image_path = join("BUILD", target, toolchain, binary_name)
     else:
-        image_path = join(ROOT, "cmake_build", target, "develop", toolchain, binary_name)
+        image_path = join("cmake_build", target, "develop", toolchain, binary_name)
 
     return {
         "binaries": [