builtin: fix tests for int

[lcheylus]

In test_str_methods, cast numbers with voidptr first.

---

Tests OK on Linux (Debian/testing amd64) with prod mode.

• Before fix

$ ./v -stats -prod test vlib/builtin/int_test.v
---- Testing... ----
vlib/builtin/int_test.v:32:10: error: cannot cast a number to a type reference, use `nil` or a voidptr cast first: `&Type(voidptr(123))`
   30 |     assert voidptr(-1).str() == '0xffffffffffffffff'
   31 |     assert voidptr(1).str() == '0x1'
   32 |     assert (&u8(-1)).str() == 'ffffffffffffffff'
      |             ~~~~~~~
   33 |     assert (&u8(1)).str() == '1'
   34 |     assert byteptr(-1).str() == '0xffffffffffffffff'
vlib/builtin/int_test.v:33:10: error: cannot cast a number to a type reference, use `nil` or a voidptr cast first: `&Type(voidptr(123))`
   31 |     assert voidptr(1).str() == '0x1'
   32 |     assert (&u8(-1)).str() == 'ffffffffffffffff'
   33 |     assert (&u8(1)).str() == '1'
      |             ~~~~~~
   34 |     assert byteptr(-1).str() == '0xffffffffffffffff'
   35 |     assert byteptr(1).str() == '0x1'
checker summary: 2 V errors, 0 V warnings, 0 V notices

----

• With fix

$ ./v -stats -prod test vlib/builtin/int_test.v
---- Testing... ----
        V  source  code size:      30118 lines,     139255 tokens,     808034 bytes,   288 types,    12 modules,   133 files
generated  target  code size:      15188 lines,     609296 bytes
compilation took: 6168.986 ms, compilation speed: 4882 vlines/s, cgen threads: 7
running tests in: /home/fox/dev/vlang.git/vlib/builtin/int_test.v
      OK    [ 1/18]     0.000 ms     4 asserts | main.test_const()
      OK    [ 2/18]     0.005 ms    25 asserts | main.test_str_methods()
      OK    [ 3/18]     0.001 ms     6 asserts | main.test_and_precedence()
      OK    [ 4/18]     0.000 ms     6 asserts | main.test_or_precedence()
      OK    [ 5/18]     0.000 ms     6 asserts | main.test_xor_precedence()
      OK    [ 6/18]     0.000 ms     2 asserts | main.test_left_shift_precedence()
      OK    [ 7/18]     0.000 ms     2 asserts | main.test_right_shift_precedence()
0
7
6
      OK    [ 8/18]     0.000 ms     1 assert  | main.test_i8_print()
      OK    [ 9/18]     0.000 ms     1 assert  | main.test_int_alias()
      OK    [10/18]     0.001 ms    13 asserts | main.test_hex()
      OK    [11/18]     0.001 ms     8 asserts | main.test_bin()
      OK    [12/18]     0.000 ms     9 asserts | main.test_oct()
      OK    [13/18]     0.000 ms    11 asserts | main.test_num_separator()
      OK    [14/18]     0.000 ms     6 asserts | main.test_int_decl()
      OK    [15/18]     0.010 ms    31 asserts | main.test_int_to_hex()
      OK    [16/18]     0.000 ms     3 asserts | main.test_repeat()
      OK    [17/18]     0.000 ms     8 asserts | main.test_int_min()
      OK    [18/18]     0.000 ms     8 asserts | main.test_int_max()
     Summary for running V tests in "int_test.v": 150 passed, 150 total. Elapsed time: 0 ms.

 OK    6183.743 ms vlib/builtin/int_test.v
----
Summary for all V _test.v files: 1 passed, 1 total. Elapsed time: 6184 ms, on 1 job. Comptime: 0 ms. Runtime: 6183 ms.

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