Testsuite: compatibility fix for older MATLAB

addpath does not deal well with relative path on older MATLABs.

Makefile

@@ -11,7 +11,7 @@ check-octave: m-unit-tests/src/mtest.m
 	$(OCTAVE) --no-init-file --silent --no-history --eval "addpath([pwd() '/../m-unit-tests/src']); runalltests" && [ ! -f ./failed ] && [ -f ./pass ]
 
 check-matlab: m-unit-tests/src/mtest.m
-	@$(MATLAB)  -nosplash -nodisplay -r "addpath m-unit-tests/src; cd tests; runalltests; quit" && [ ! -f ./tests/failed ] && [ -f ./tests/pass ]
+	@$(MATLAB)  -nosplash -nodisplay -r "addpath([pwd '/m-unit-tests/src']); cd tests; runalltests; quit" && [ ! -f ./tests/failed ] && [ -f ./tests/pass ]
 
 check-clean:
 	rm -f tests/*_test_*.m tests/*.csv tests/*.xls tests/*.xlsx tests/*.mat tests/failed tests/datafile_for_test