Fix linking and misparse of screenpooltest

screenpooltest.cpp:73:38: warning: empty parentheses were disambiguated as a function declaration [-Wvexing-parse]
   73 |     ScreenPoolTester screenPoolTester();
      |                                      ^~
screenpooltest.cpp:73:38: note: remove parentheses to default-initialize a variable
   73 |     ScreenPoolTester screenPoolTester();
      |                                      ^~
      |                                      --
screenpooltest.cpp:73:38: note: or replace parentheses with braces to value-initialize a variable

(cherry picked from commit 2785774fc089d56986497b950035f82001e434f0)