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.