loadNXDTestFile: use `compile` for better tracebacks on errors

When using compile with the actual file path, we can have better tracebacks
in case of errors.

before:

    Traceback (most recent call last):
      File "/srv/slapgrid/slappart3/srv/runner/software/9544feb19475590d240ba2d32743c0a0/bin/nxdtest", line 22, in <module>
        sys.exit(nxdtest.main())
      File "/srv/slapgrid/slappart3/srv/runner/software/9544feb19475590d240ba2d32743c0a0/parts/nxdtest/nxdtest/__init__.py", line 142, in main
        tenv = loadNXDTestFile('.nxdtest')
      File "/srv/slapgrid/slappart3/srv/runner/software/9544feb19475590d240ba2d32743c0a0/parts/nxdtest/nxdtest/__init__.py", line 75, in loadNXDTestFile
        six.exec_(src, g)
      File "<string>", line 77, in <module>
    NameError: name 'Pylint' is not defined

after:

    Traceback (most recent call last):
      File "/srv/slapgrid/slappart3/srv/runner/software/9544feb19475590d240ba2d32743c0a0/bin/nxdtest", line 22, in <module>
        sys.exit(nxdtest.main())
      File "/srv/slapgrid/slappart3/srv/runner/software/9544feb19475590d240ba2d32743c0a0/parts/nxdtest/nxdtest/__init__.py", line 142, in main
        tenv = loadNXDTestFile('.nxdtest')
      File "/srv/slapgrid/slappart3/srv/runner/software/9544feb19475590d240ba2d32743c0a0/parts/nxdtest/nxdtest/__init__.py", line 75, in loadNXDTestFile
        six.exec_(compile(src, os.path.realpath(path), 'exec'), g)
      File "/srv/slapgrid/slappart3/srv/runner/instance/slappart8/var/nxdtest/.nxdtest", line 77, in <module>
        summaryf=Pylint.summary,
    NameError: name 'Pylint' is not defined