Tidy default allocator features

[febo]

Problem

The default_allocator! macro can be used in different contexts, e.g., when running cargo build-sbf, cargo test-sbf or cargo clippy. Currently it is gated by #[cfg(target_os = "solana")], which works fine for cargo build-sbf and cargo test-sbf but it is not present for cargo clippy. This creates a compilation error when the program crate is no_std.

PR #125 solved the issue for clippy, but it is currently generating a warning since there is a misplaced feature = "std" test.

Solution

Use #[cfg(not(any(target_os = "solana", test)))] to gate the placeholder default allocator instead of using feature = "std" test.

This is an alternative solution to PR #128.