Test pb-long without BI support and fix found issues

[jcready]

Initially my goal was simply to improve code coverage, but in doing so I discovered a number of issues related to pb-long when the test environment did not have BigInt support.

Issues discovered and fixed with pb-long w/o BI support:

1. PbLong.from() did not throw when the string value was too small "-9223372036854775809", "-18446744073709551616"

2. protobuf-ts/packages/runtime/spec/pb-long.spec.ts

   Lines 133 to 134 in 2f0ed59

   	let maxSignedPlusOneStr = "9223372036854775808"
   	expect(() => PbLong.from(maxSignedPlusOneStr)).toThrowError();

   Expected function to throw an Error.

3. protobuf-ts/packages/runtime/spec/pb-long.spec.ts

   Lines 37 to 40 in 2f0ed59

   	it('should toString() max unsigned int64', function () {
   	let uLong = new PbULong(-1, -1);
   	expect(uLong.toString()).toBe('18446744073709551615');
   	});

   Expected '-1' to be '18446744073709551615'.

4. protobuf-ts/packages/runtime/spec/pb-long.spec.ts

   Lines 109 to 112 in 2f0ed59

   	it('should toString() min signed int64', function () {
   	let uLong = new PbLong(0, -2147483648);
   	expect(uLong.toString()).toBe('-9223372036854775808');
   	});

   Expected '--9223372036854776000' to be '-9223372036854775808'.

This change fixes those issues, ensures all pb-long tests are performed with and without BI support so the testing runtime environment doesn't strictly limit what can be tested. It also improves the test coverage for pb-long.

[timostamm]

Thank you for the fix!

Pushed up one commit that moves the test setup into a describeWithAndWithoutBISupport() function.