fix(std/testing): formatting bigint (#4626)

std/testing/format.ts

@@ -115,6 +115,10 @@ function printSymbol(val: symbol): string {
   return symbolToString.call(val).replace(SYMBOL_REGEXP, "Symbol($1)");
 }
 
+function printBigInt(val: bigint): string {
+  return val.toString() + "n";
+}
+
 function printError(val: Error): string {
   return "[" + errorToString.call(val) + "]";
 }
@@ -155,6 +159,9 @@ function printBasicValue(
   if (typeOf === "symbol") {
     return printSymbol(val);
   }
+  if (typeOf === "bigint") {
+    return printBigInt(val);
+  }
 
   const toStringed = toString.call(val);
 

std/testing/format_test.ts

@@ -580,6 +580,14 @@ test({
   },
 });
 
+test({
+  name: "prints a bigint",
+  fn(): void {
+    const val = 12345n;
+    assertEquals(format(val), "12345n");
+  },
+});
+
 test({
   name: "prints undefined",
   fn(): void {