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=(