Expands the pop-up disabling in drcachesim's launcher to warnings and errors as well as asserts.
Applies the same MSVC message box disabling to the tests by adding it to INIT() in tools.h.
Tested on win32.tls.exe which asserts on Win10.
Also fixes the assert in win32.tls on win8+ by switching from raw NtCreateThread to high-level thread creation, with an allowance added to the core to let this test in DEBUG target our nudge handler. This also makes it easy to enable the test for 64-bit mode after all that low-level code is removed.
Issue: #4067