Fixed command not found error during tests on windows

Added missing cmdSuffix in tests. Previously it was used only in the actual code. This fixes quite a number of tests on Windows.
If you need to add the extension when calling "new command" don't you need to add it here as well? (Another case where we probably just want jujuc.NewCommand to DTRT and add it when necessary.)
Otherwise there is a disconnect, because here we pass "random" but get "random.exe" attempted, but later you claim we have to pass "foo.exe" everywhere.

As for the test itself, I'm happy to just have "random(.exe)?" with a comment about "// the error message contains .exe on Windows".