Make it clear which functions that are intended to be translation unit local

Do not share functions that are meant to be translation unit local with
other translation units. Use internal linkage for those consistently.

src/test/main_tests.cpp

@@ -58,8 +58,8 @@ BOOST_AUTO_TEST_CASE(subsidy_limit_test)
     BOOST_CHECK_EQUAL(nSum, CAmount{2099999997690000});
 }
 
-bool ReturnFalse() { return false; }
-bool ReturnTrue() { return true; }
+static bool ReturnFalse() { return false; }
+static bool ReturnTrue() { return true; }
 
 BOOST_AUTO_TEST_CASE(test_combiner_all)
 {