Cmake tests: Make foreach() command robust

Cmake tests: Make foreach() command robust

Cmake's foreach command includes forms
  foreach(<loop_var> IN LISTS <lists>)
  foreach(<loop_var> IN ITEMS <items>)
  foreach(<loop_var> RANGE ...)
We get the lines to be parsed by
  file(STRINGS "${filepath}" lines)
If in the parsed lyx-file there is a line
containing only the single word 'IN',  or 'RANGE', then
the command
  foreach(_l ${lines})
can create a syntax error (at least with cmake1.16)

In fact, in file pl_Additional.lyx:12913 happens to have
such a beast.

(cherry picked from commit 7b44280da4fc015a4a930c311cc6073cebacf738)