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/appointment-022.sh

@@ -1,5 +1,7 @@
 #!/bin/sh
 
+. ./test-init.sh
+
 "$CALCURSE" --read-only -D "$DATA_DIR"/ -c "$DATA_DIR/apts-appointment-022" \
   -d02/23/2013 2>errors && exit 1
 grep -Fq 'date error in appointment' errors