Implement test initialization properly

Make test-init.sh work even if tests are executed from another
directory.

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

test/run-test-001.sh

@@ -1,6 +1,6 @@
 #!/bin/sh
 
-. ./test-init.sh
+. "${TEST_INIT:-./test-init.sh}"
 
 if [ "$1" = 'actual' -o "$1" = 'expected' ]; then
   echo 42