[lld-link] Tweak winsysroottest.test to have passing links on happy path

Previously, the test checked for a "undefined symbol" error
(instead of the "could not open std*.lib" which would happen without
the flag).

Instead, use /entry: so that the link succeeds.

No behavior change, but maybe makes the test a bit easier to understand.

Differential Revision: https://reviews.llvm.org/D121553