test-lib.sh: unset some more TODOTXT env vars for extra safety

While preparing a setup where env vars overload config file,
I had the bad experience that my actual todo.txt was messed up
by the testsuite so it's safer to unset everything.

Even so, some tests could have failed if some TODOTXT_ were set
in the user environment, for example:
export TODOTXT_PLAIN=1
./t1200-pri.sh

Since test-lib.sh does not depend on bash currently, calculate
the list of variables to be unset using set|sed.

Signed-off-by: Emil Sit <sit@emilsit.net>

tests/test-lib.sh

@@ -37,7 +37,9 @@ VISUAL=:
 unset CDPATH
 
 # Protect ourselves from using predefined TODOTXT_CFG_FILE
-unset TODOTXT_CFG_FILE
+unset TODOTXT_CFG_FILE $(set|sed '/^TODOTXT_/!d;s/=.*//')
+# To prevent any damage if someone has still those exported somehow in his env:
+unset TODO_FILE DONE_FILE REPORT_FILE TMP_FILE
 
 # Each test should start with something like this, after copyright notices:
 #