Fix bug in source file test.
Fix bug in source file test.

When looking for a source file, it was assumed that the current working
directory did not contain the character '.'. If the working directory did
contain a '.', this test would fail.

paulromano/openmc,sxds/opemmc,samuelshaner/openmc,bhermanmit/openmc,walshjon/openmc,mjlong/openmc,paulromano/openmc,johnnyliu27/openmc,shikhar413/openmc,smharper/openmc,smharper/openmc,walshjon/openmc,smharper/openmc,mjlong/openmc,amandalund/openmc,lilulu/openmc,wbinventor/openmc,wbinventor/openmc,liangjg/openmc,shenqicang/openmc,liangjg/openmc,mit-crpg/openmc,kellyrowland/openmc,wbinventor/openmc,shikhar413/openmc,lilulu/openmc,wbinventor/openmc,sxds/opemmc,samuelshaner/openmc,mit-crpg/openmc,amandalund/openmc,liangjg/openmc,liangjg/openmc,paulromano/openmc,samuelshaner/openmc,johnnyliu27/openmc,mit-crpg/openmc,nhorelik/openmc,amandalund/openmc,johnnyliu27/openmc,samuelshaner/openmc,shikhar413/openmc,mit-crpg/openmc,walshjon/openmc,bhermanmit/openmc,shenqicang/openmc,paulromano/openmc,kellyrowland/openmc,smharper/openmc,lilulu/openmc,amandalund/openmc,nhorelik/openmc,johnnyliu27/openmc,walshjon/openmc,shikhar413/openmc