Do not use readlink(1) in tests

Avoid using readlink(1) which is not POSIX-compatible; moreover,
`readlink -f` is not available on Mac OS by default.

Instead, always convert $CALCURSE to an absolute path (that may or may
not be canonical, i.e., be a symlink or contain ../ as component) in
test-init.sh by prepending the current working directory if the original
path is relative. While not fully equivalent to `readlink -f`, this
should be good enough for use in our tests.

Signed-off-by: Lukas Fleischer <lfleischer@calcurse.org>

test/test-init.sh

@@ -2,3 +2,8 @@
 
 CALCURSE=${CALCURSE:-../src/calcurse}
 DATA_DIR=${DATA_DIR:-data/}
+
+case "$CALCURSE" in
+	/*) ;;
+	*) CALCURSE="${PWD}/${CALCURSE}" ;;
+esac