Raise an error if variables are not defined.

tests/Modifiers_-_--force.test

@@ -3,8 +3,8 @@
 # Check that the progress bar is produced when "--force" is used, and is not
 # when it is not, providing stderr is a file.
 
-# Dummy assignments for "shellcheck".
-testSubject="${testSubject:-false}"; workFile1="${workFile1:-.tmp1}";
+true "${testSubject:?not set - call this from 'make check'}"
+true "${workFile1:?not set - call this from 'make check'}"
 
 dd if=/dev/zero bs=100 count=1 2>/dev/null \
 | "${testSubject}" -f -p -i 0.1 -L 100 >/dev/null 2>"${workFile1}"