Suppress -Winteger-overflow compiler warning

The tests intentionally tests overflow behaviour.

strings_test.cc:56:29: error: overflow in expression; result is -2147483648 with type 'int' [-Werror,-Winteger-overflow]
   56 |     ASSERT_EQ(ToHexString(1 + INT_MAX), "INT_MIN");
      |     ~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~

Test: presubmit
Change-Id: I4da2f67db9b01bc5b73153395b047487c7391a18