Fix BigInt#testBit to throw ArithmeticException when n is negative.

The `_bigInteger` field, a mutable value initialized by `BigInt#bigInteger`, is accessed directly by `BigInt#testBit` when the input `n` is negative.
As a result, `testBit` currently throws a `NullPointerException`.

The expected behavior is for `testBit` to throw an `ArithmeticException` when `n` is negative, as specified by the underlying `java.math.BigInteger` API:
https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/math/BigInteger.html#testBit(int)

This issue was not detected in `BigIntTest` because `_bigInteger` is initialized by earlier tests.
Additionally, using `BigInt(42).testBit(-3)` for testing does not reproduce the issue, as `BigInt(42)` uses a cached instance where `_bigInteger` is already initialized.

See: scala-native/scala-native#2897

Author: tanishiking

library/src/scala/math/BigInt.scala

@@ -379,7 +379,7 @@ final class BigInt private (private var _bigInteger: BigInteger, private val _lo
       else if (_long < 0) BigInt(-1)
       else BigInt(0) // for _long >= 0
     } else BigInt(this.bigInteger.shiftRight(n))
-  
+
   /** Bitwise and of BigInts
    */
   def &(that: BigInt): BigInt =
@@ -501,7 +501,7 @@ final class BigInt private (private var _bigInteger: BigInteger, private val _lo
         (_long & (1L << n)) != 0
       else
         _long < 0 // give the sign bit
-    } else _bigInteger.testBit(n)
+    } else this.bigInteger.testBit(n)
 
   /** Returns a BigInt whose value is equivalent to this BigInt with the designated bit set.
    */