Fix BigInteger outerloop test

src/libraries/System.Runtime.Numerics/tests/BigInteger/BigIntegerToStringTests.cs

@@ -521,22 +521,23 @@ public static void ToString_InvalidFormat_ThrowsFormatException()
             Assert.Throws<FormatException>(() => b.ToString("G000001000000000"));
         }
 
-        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.Is64BitProcess))] // Requires a lot of memory
-        [OuterLoop("Takes a long time, allocates a lot of memory")]
-        [SkipOnMono("Frequently throws OOM on Mono")]
+        [Fact]
         public static void ToString_ValidLargeFormat()
         {
             BigInteger b = new BigInteger(123456789000m);
 
             // Format precision limit is 999_999_999 (9 digits). Anything larger should throw.
+            // We use TryFormat rather than ToString to avoid excessive memory usage.
 
-            // Check ParseFormatSpecifier in FormatProvider.Number.cs with `E` format
-            b.ToString("E999999999"); // Should not throw
-            b.ToString("E00000999999999"); // Should not throw
+            // Check ParseFormatSpecifier in FormatProvider.Number.cs with `E` format.
+            // Currently disabled since these would still allocate a 2GB buffer before
+            // returning, leading to OOM in CI.
+            //Assert.False(b.TryFormat(Span<char>.Empty, out _, format: "E999999999")); // Should not throw
+            //Assert.False(b.TryFormat(Span<char>.Empty, out _, format: "E00000999999999")); // Should not throw
 
             // Check ParseFormatSpecifier in Number.BigInteger.cs with `G` format
-            b.ToString("G999999999"); // Should not throw
-            b.ToString("G00000999999999"); // Should not throw
+            Assert.False(b.TryFormat(Span<char>.Empty, out _, format: "G999999999")); // Should not throw
+            Assert.False(b.TryFormat(Span<char>.Empty, out _, format: "G00000999999999")); // Should not throw
         }
 
         private static void RunSimpleProviderToStringTests(Random random, string format, NumberFormatInfo provider, int precision, StringFormatter formatter)

src/libraries/System.Runtime.Numerics/tests/BigInteger/GetBitLengthTests.cs

@@ -44,8 +44,10 @@ public static void RunGetBitLengthTests()
         public static void RunGetBitLengthTestsLarge()
         {
             // Very large cases
-            VerifyGetBitLength(BigInteger.One << 32 << int.MaxValue, int.MaxValue + 32L + 1, 1);
-            VerifyGetBitLength(BigInteger.One << 64 << int.MaxValue, int.MaxValue + 64L + 1, 1);
+            // Values which are large but beneath the upper bound of
+            // (2^31) - 1 bits and which should not cause OOM in CI.
+            VerifyGetBitLength(BigInteger.One << 32 << (1 << 24), (1 << 24) + 32L + 1, 1);
+            VerifyGetBitLength(BigInteger.One << 64 << (1 << 24), (1 << 24) + 64L + 1, 1);
         }
 
         private static void VerifyLoopGetBitLength(Random random, bool isSmall)