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/io-006.sh

@@ -3,7 +3,6 @@
 . "${TEST_INIT:-./test-init.sh}"
 dir=$(mktemp -d)
 
-CALCURSE=$(readlink -f "$CALCURSE")
 cd "$dir"
 (unset -v HOME XDG_DATA_HOME XDG_CONFIG_HOME; "$CALCURSE" -a)
 [ -f "$dir/.calcurse/apts" ] && [ -f "$dir/.calcurse/conf" ] && failed=0 || failed=1