LibCrypto: Fix subtracting two negative `SignedBigInteger`s

Currently, we get the following results

    -1 - -2 = -1
    -2 - -1 =  1

Correct would be:

    -1 - -2 =  1
    -2 - -1 = -1

This was already attempted to be fixed in 7ed8970, but that change was
incorrect. This directly translates to LibJS BigInts having the same
incorrect behavior - it even was tested.

1 files changed, 9 insertions, 0 deletions

diff --git a/Tests/LibCrypto/TestBigInteger.cpp b/Tests/LibCrypto/TestBigInteger.cpp
index 36b3098b21..504fb4eab1 100644
--- a/Tests/LibCrypto/TestBigInteger.cpp
+++ b/Tests/LibCrypto/TestBigInteger.cpp
@@ -543,6 +543,15 @@ TEST_CASE(test_signed_subtraction_simple_subtraction_negative_result)
     EXPECT_EQ(num1.minus(num2), Crypto::SignedBigInteger { -20 });
 }
 
+TEST_CASE(test_signed_subtraction_both_negative)
+{
+    Crypto::SignedBigInteger num1(-50);
+    Crypto::SignedBigInteger num2(-70);
+
+    EXPECT_EQ(num1.minus(num2), Crypto::SignedBigInteger { 20 });
+    EXPECT_EQ(num2.minus(num1), Crypto::SignedBigInteger { -20 });
+}
+
 TEST_CASE(test_signed_subtraction_simple_subtraction_with_borrow)
 {
     Crypto::SignedBigInteger num1(Crypto::UnsignedBigInteger { UINT32_MAX });