Prevent path fallback for mono-sgen and pedump

Test updated to allow "mono-sgen" and "pedump" binaries to "walk" the parent
directory to discover the install location of the mono runtime in the same
manner as the "mono" binary.
Merge pull request #590 from qhalid/mono-2-10

Xamarin Bug 10879: Use of mono-sgen causes mono path detection fallback (2.10 backport)