[Tests] truncate: cover BigInt inputs

The implementation already accepts BigInt
(per the spec, `truncate` is a purely-mathematical operation that produces a mathematical integer for any BigInt input - the identity), but no test exercised that path.
Adds five assertions covering 0n, ±5n, and arbitrary-precision ±10**29.

Author: ljharb

test/methods/truncate.js

@@ -19,4 +19,24 @@ module.exports = function (t, year, truncate) {
 	t.equal(truncate(1.1), 1, '1.1 truncates to 1');
 	t.equal(truncate(0), 0, '+0 truncates to +0');
 	t.equal(truncate(-0), 0, '-0 truncates to +0');
+
+	t.test('has bigints', { skip: v.bigints.length === 0 }, function (st) {
+		// BigInt support: integer-valued, so truncate is the identity, and
+		// arbitrarily-large values are preserved without precision loss.
+		st.equal(truncate(BigInt(0)), BigInt(0), '0n is unchanged');
+		st.equal(truncate(BigInt(5)), BigInt(5), '5n is unchanged');
+		st.equal(truncate(BigInt(-5)), BigInt(-5), '-5n is unchanged');
+		st.equal(
+			truncate(BigInt('123456789012345678901234567890')),
+			BigInt('123456789012345678901234567890'),
+			'arbitrary-precision BigInt is preserved'
+		);
+		st.equal(
+			truncate(BigInt('-123456789012345678901234567890')),
+			BigInt('-123456789012345678901234567890'),
+			'arbitrary-precision negative BigInt is preserved'
+		);
+
+		st.end();
+	});
 };