is_exe_in_path() may be called multiple times with the same executable

name, particularly when running from unit tests. Since an executable is

unlikely to appear or disappear within the lifetime of a process run in

a way that impacts RBTools, it's not worth re-scanning the whole path

every time we need to know if an executable is available.
This change adds caching behavior to is_exe_in_path(). Now, a repeated

call with the same executable name (with or without a .exe file

extension) will return the prior cached result.

All unit tests pass.