fix ProjectRootDirectory lookup

When the tests are run in TeamCity, the assembly location is this:

Z:\Builds\work\bcdbe6b8cc677a49\src\Lucene.Net.Tests\bin\Release

the "\Builds\" part was tripping up path look up logic as it would test for presence for "build" (case invariant) and then expect that to be "\build\". Rewrote it so that it checks first for "\build\" (instead of "build") and then for "\bin\" as a fallback.