test/: Add default values for configuration variables

Add a small helper script called "test-init.sh" that is sourced in each
test case and initializes the CALCURSE and DATA_DIR environment
variables.

Update the test suite README to point out that setting these environment
variables is no longer required.

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

test/test-init.sh

@@ -0,0 +1,4 @@
+#!/bin/sh
+
+CALCURSE=${CALCURSE:-../src/calcurse}
+DATA_DIR=${DATA_DIR:-data/}