[tools] Fixed mono-shlib-cop test

MONO_PATH was missing when running the tests.

Use "libMonoPosixHelper.so" in local directory when checking if DllImport of a development library generates a warning (libglib2.0-dev might not be installed, but libMonoPosixHelper is always built with mono)
Merge pull request #1141 from akoeplinger/fix-mono-shlib-cop

[tools] Fixed mono-shlib-cop test