Don't test panic=unwind in panic_main.rs on Fuchsia
````@Enselic```` added a few new test conditions to tests/ui/panics/panic-main.rs in rust-lang/rust#142304, but it is unfortunately causing the test to fail for Fuchsia with the `panic=unwind` modes since we compile Rust for Fuchsia with `panic=abort`. This patch just ignores the test for Fuchsia.
Note that this test might also need to filter out a few other platforms, since another panicking test we exclude from Fuchsia https://github.com/rust-lang/rust/blob/master/tests/ui/panics/runtime-switch.rs also excludes running on msvc, android, openbsd, and wasm, but I'm not familiar with those platforms so I didn't want to add them here.
cc ````@compile-errors,```` who reviewed the initial PR
5e2416fd76