test/: Use correct calcurse binary and data dir

We really don't want to use the calcurse(1) binary from "$PATH" here.
Introduce an environment variable "$CALCURSE" which can be used specify
the binary to use and set this variable to the binary located in the
build directory by default. Similarly, add another environment variable
"$DATA_DIR" that can be used to specify a data directory.

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

test/todo-002.sh

@@ -1,10 +1,10 @@
 #!/bin/sh
 
 if [ "$1" = 'actual' ]; then
-  calcurse -D data/ -t3
+  "$CALCURSE" -D "$DATA_DIR"/ -t3
 elif [ "$1" = 'expected' ]; then
   echo 'to do:'
-  sed -n 's/^\[3\] \(.*\)/3. \1/p' data/todo
+  sed -n 's/^\[3\] \(.*\)/3. \1/p' "$DATA_DIR"/todo
 else
   ./run-test "$0"
 fi