Fix test in test-spawn.lua (#1298)

Copy-paste-y I was checking for the wrong result. Another unrelated
problem with the test runner caused me to miss this. Sorry!

Signed-off-by: Uli Schlachter <psychon@znc.in>

tests/test-spawn.lua

@@ -30,7 +30,7 @@ local steps = {
     assert(string.find(error_message, 'Text was empty'), error_message)
 
     error_message = spawn("")
-    assert(string.find(error_message, 'Text was empty'), error_message)
+    assert(string.find(error_message, 'No command to execute'), error_message)
 
     error_message = spawn{}
     assert(string.find(error_message, 'There is nothing to execute'), error_message)