RunWithFakeThreshold with DisableParallelization

[kzrnm]

Fix #103247

Non-reproducible assertion errors often occur when tests are run in parallel. I suspect it is due to RunWithFakeThreshold affecting other tests. I hope that applying DisableParallelization to tests that use RunWithFakeThreshold will avoid these errors.

Detail

runtime/src/libraries/System.Runtime.Numerics/src/System/Numerics/BigIntegerCalculator.SquMul.cs

Line 333 in 6927fea

Debug.Assert(right.Length < MultiplyKaratsubaThreshold);

Multiplication of BigInteger has this assersion.

runtime/src/libraries/System.Runtime.Numerics/src/System/Numerics/BigIntegerCalculator.SquMul.cs

Lines 152 to 158 in 6927fea

	#if DEBUG
	// Mutable for unit testing...
	internal static
	#else
	internal const
	#endif
	int MultiplyKaratsubaThreshold = 32;

MultiplyKaratsubaThreshold is 32 and the field is immutable in normal execution.

runtime/src/libraries/System.Runtime.Numerics/tests/BigInteger/multiply.cs

Lines 36 to 50 in 6927fea

	[Fact]
	public static void RunMultiply_TwoLargeBigIntegers_Threshold()
	{
	// Again, with lower threshold
	BigIntTools.Utils.RunWithFakeThreshold(BigIntegerCalculator.SquareThreshold, 8, () =>
	BigIntTools.Utils.RunWithFakeThreshold(BigIntegerCalculator.MultiplyKaratsubaThreshold, 8, RunMultiply_TwoLargeBigIntegers)
	);
	// Again, with lower threshold
	BigIntTools.Utils.RunWithFakeThreshold(BigIntegerCalculator.SquareThreshold, 8, () =>
	BigIntTools.Utils.RunWithFakeThreshold(BigIntegerCalculator.MultiplyKaratsubaThreshold, 8, () =>
	BigIntTools.Utils.RunWithFakeThreshold(BigIntegerCalculator.StackAllocThreshold, 8, RunMultiply_TwoLargeBigIntegers)
	)
	);
	}

RunWithFakeThreshold rewrites MultiplyKaratsubaThreshold to 8.

Flow of assertion (my guess)

For the sake of explanation, let {a} denote a Span of length a. For example, {8} is a Span of length 8.

Normal

1. [testA] BigIntegerCalculator.Multiply({32}, {32})
2. [testA] BigIntegerCalculator.Multiply.MultiplyKaratsuba({32}, {32})
3. [testA] BigIntegerCalculator.Multiply.Naive({16}, {16})
4. [testA] Assert(16 < MultiplyKaratsubaThreshold = 32)

Parallel

1. [testA] BigIntegerCalculator.Multiply({32}, {32})
2. [testA] BigIntegerCalculator.Multiply.MultiplyKaratsuba({32}, {32})
3. [testA] BigIntegerCalculator.Multiply.Naive({16}, {16})
4. [testB] RunWithFakeThreshold(8,...): MultiplyKaratsubaThreshold = 8 ‼️
5. [testA] Assert(16 < MultiplyKaratsubaThreshold = 8) ⚠️