To properly test tier-up in the V8 test suite, change the test variant
previously called --turbo-nci-as-highest-tier to
--turbo-nci-as-midtier.  As a midtier (between ignition and turbofan),
all major parts of the NCI pipeline (codegen, caching inside the same
native context, tier-up) are exercised by test suite.

Bug: v8:8888
Change-Id: Ic8ee2f3e3d72768c3869f5e0b25800dd0a5f25b7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2361462
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69501}