test(config): sandboxed config-layer selftests (#3)

20 cases: bootstrap create/idempotency, missing config, malformed JSON,
unknown keys/version/backend/environment, relative and non-canonical
dataRoot, filesystem root, home dir, ancestor-of-config, control chars,
symlinked config file, env export resolution, validation-writes-nothing.

Closes #3

scripts/test-config.sh

@@ -112,8 +112,8 @@ cfg ctrlchar.json '{"configVersion":1,"environment":"development","dataRoot":"'$
 expect_exit "control character in provider exits 2" 2 -- \
     env MOSAIC_CONFIG="$SANDBOX/ctrlchar.json" $CONFIG_OP validate
 
-cfg symlink.json
-ln -sf "$SANDBOX/invalid.json" "$SANDBOX/symlink.json" 2>/dev/null
+cfg symlink.json 'placeholder'
+ln -sf "$SANDBOX/invalid.json" "$SANDBOX/symlink.json"
 expect_exit "symlinked config file exits 2" 2 -- \
     env MOSAIC_CONFIG="$SANDBOX/symlink.json" $CONFIG_OP validate