Rollup merge of #111042 - Zalathar:no-coverage, r=wesleywiser

Add `#[no_coverage]` to the test harness's `fn main`

There are two main motivations for adding `#[no_coverage]` to the test harness's entry point:

- The entry point is trivial compiler-generated code that doesn't correspond to user source, and it always runs, so there's no value in instrumenting it for coverage.
- Because it has dummy spans, it causes the instrumentor implementation to emit invalid coverage mappings that confuse `llvm-cov` and result in strange coverage reports.

Fixes #110749.

Author: matthiaskrgr