Missing cli flag to control CanBlock in run-test262

[ivankra]

The two CanBlockIsFalse tests from test262 aren't reproducible using run-test262 -N as there's no way to control [[CanBlock]] property via a command-line flag:

test/built-ins/Atomics/wait/bigint/cannot-suspend-throws.js: Test262Error: Expected a TypeError to be thrown but no exception was thrown at all
test/built-ins/Atomics/wait/cannot-suspend-throws.js: Test262Error: Expected a TypeError to be thrown but no exception was thrown at all

d8 has --no-can-block flag - I'd like to have an equivalent in run-test262 for its -N mode.

(Also in quickjs: bellard/quickjs#501)

[saghul]

Sounds good, would you like to send a PR?

[ivankra]

Sure, it's a simple change.