Fix old record syntax warning in test's Main file

[mattpolzin]

Warning: DEPRECATED: old record update syntax.
  Use "{ f := v } p" instead of "record { f = v } p"
  and "{ f $= v } p" instead of "record { f $= v } p"
Main:17:21--17:66
 13 | main = runner
 14 |   [ testPaths "lsp" allTests
 15 |   ] where
 16 |     testPaths : String -> TestPool -> TestPool
 17 |     testPaths dir = record { testCases $= map ((dir ++ "/") ++) }
                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^