math: use full import module name in tests

vlib/math/big/big_test.v

@@ -1,4 +1,4 @@
-import big
+import math.big
 
 fn test_new_big() {
 	n := big.new()

vlib/math/fractions/approximations_test.v

@@ -1,7 +1,7 @@
 // Copyright (c) 2019-2020 Alexander Medvednikov. All rights reserved.
 // Use of this source code is governed by an MIT license
 // that can be found in the LICENSE file.
-import fractions
+import math.fractions
 import math
 
 fn test_half() {