tests: Fix a wrong value of $PREBUFFER in a test, and add checks to prevent this from recurring.

Discussed: https://github.com/zsh-users/zsh-syntax-highlighting/pull/706#issuecomment-642072978

highlighters/main/test-data/path_prefix3.zsh

@@ -30,7 +30,7 @@
 # Assumes that '/bin/sh' exists and '/bin/s' does not exist.
 # Related to path_prefix.zsh
 
-PREBUFFER='ls \'
+PREBUFFER=$'ls \\\n'
 BUFFER='/bin/s'
 
 expected_region_highlight=(

tests/test-highlighting.zsh

@@ -145,6 +145,7 @@ run_test_internal() {
 
     # Check the data declares $PREBUFFER or $BUFFER.
     [[ -z $PREBUFFER && -z $BUFFER ]] && { echo >&2 "Bail out! On ${(qq)ARG}: Either 'PREBUFFER' or 'BUFFER' must be declared and non-blank"; return ${RETURN:=1}; }
+    [[ $PREBUFFER == (''|*$'\n') ]] || { echo >&2 "Bail out! On ${(qq)ARG}: PREBUFFER=${(qqqq)PREBUFFER} doesn't end with a newline"; return ${RETURN:=1}; }
 
     # Set sane defaults for ZLE variables
     : ${CURSOR=$#BUFFER} ${PENDING=0} ${WIDGET=z-sy-h-test-harness-test-widget}