Replace backticks in config with $(...)

This applies e24777fa2c to todo.cfg, too, and adds a verification to the existing test.

Note: Because todo.cfg is preprocessed by test-lib, we use the original config from the working copy, not the copy inside the test fixture.

todo.cfg

@@ -2,7 +2,7 @@
 
 # Your todo.txt directory
 #export TODO_DIR="/Users/gina/Documents/todo"
-export TODO_DIR=`dirname "$0"`
+export TODO_DIR=$(dirname "$0")
 
 # Your todo/done/report.txt locations
 export TODO_FILE="$TODO_DIR/todo.txt"