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/range-002.sh

@@ -1,5 +1,7 @@
 #!/bin/sh
 
+. ./test-init.sh
+
 if [ ! -x "$(command -v faketime)" ]; then
   echo "libfaketime not found - skipping $0..."
   exit 0