Convert TF_HEADER to YAML

Should have been for a long time now.

tests/TF_HEADER

@@ -3,8 +3,8 @@
 
 . "$(sfpath)"
 
-echo "version = $SHELLFU_VERSION"
-echo "bin = $(which sfpath)"
-echo "SHELLFU_DIR = $SHELLFU_DIR"
-echo "SHELLFU_INCLUDE = $SHELLFU_INCLUDE"
-echo "SHELLFU_PATH = $SHELLFU_PATH"
+echo "version: ${SHELLFU_VERSION:-null}"
+echo "bin: $(which sfpath)"
+echo "SHELLFU_DIR: ${SHELLFU_DIR:-null}"
+echo "SHELLFU_INCLUDE: ${SHELLFU_INCLUDE:-null}"
+echo "SHELLFU_PATH: ${SHELLFU_PATH:-null}"