What exact version of the toolchain do you have? This looks like some sort of path name problem. Did you build this directly in the source tree? You could try checking the makefile to see why the path name is wrong.
There is no Makefile rule to clean only the test/dejagnu files. If you want to rerun the tests, you only need to remove the corresponding stamp files. If you want to recover the disk space, then something “rm -rf build//build-gcc--stage2/gcc/testsuite” should work.