test(screen): still match by full row when {MATCH:} is present (#34437)

Add '^' and '$' around the pattern. This makes it less likely to make
mistakes of when writing tests with {MATCH:}, as most such tests have
text before and after {MATCH:}.

test/functional/ui/screen.lua

@@ -586,6 +586,7 @@ function Screen:expect(expected, attr_ids, ...)
             after = after:sub(e + 1)
           end
         end
+        pat = pat and '^' .. pat .. '$'
         if row ~= actual_rows[i] and (not pat or not actual_rows[i]:match(pat)) then
           msg_expected_rows[i] = '*' .. msg_expected_rows[i]
           if i <= #actual_rows then