Fix: ASM test compilation when binaries are missing

[mikemaccana]

Problem

Four ASM examples (transfer-sol, create-account, checking-accounts, hello-solana) failed to compile whenever the SBPF .so artifacts were absent:

error: couldn't read `basics/transfer-sol/asm/deploy/transfer-sol-cpi.so`: No such file or directory

The tests embed the assembled program with include_bytes!, which resolves at compile time. Those binaries are gitignored build artifacts produced by sbpf build, so a fresh clone has none and the crate cannot build at all — the failure is a compile error, not a skipped test.

Solution

A build.rs per ASM crate checks for that crate's binary and sets a has_asm_binary cfg when it is present. Each test module is gated on #[cfg(all(test, has_asm_binary))]:

• Binary present (after sbpf build): the tests compile and run exactly as before.
• Binary absent: the test module is left out, the crate builds, and two cargo::warning lines point at sbpf build.

Each script also emits cargo::rerun-if-changed for its binary, so the cfg is re-evaluated as soon as the file appears rather than going stale.

Changes

• build.rs added to 4 ASM crates, each naming the binary its own crate embeds
• build = "build.rs" added to the 4 Cargo.toml files
• test modules gated #[cfg(test)] → #[cfg(all(test, has_asm_binary))]

Notes for reviewers

Two things surfaced while getting CI green, both worth knowing since they are easy to reproduce:

-D warnings promotes unexpected_cfgs to an error. Introducing a custom cfg is not enough; it has to be declared, or cargo clippy -- -D warnings fails on every crate that uses it. Each script emits cargo::rustc-check-cfg=cfg(has_asm_binary).

The binary names differ per crate. The four crates embed transfer-sol-cpi.so, create-account-asm-program.so, checking-account-asm-program.so and hello-solana-asm-program.so respectively. An earlier revision of this branch probed deploy/program.so in three of them, which would have left the cfg unset after a real sbpf build and silently switched those tests off — quieter than the bug being fixed, and worse. Each script now names the file its own crate embeds.

Testing

Both directions verified locally, not just the one CI exercises:

# CI's exact invocation
cargo clippy -- -D warnings -A clippy::diverging_sub_expression   # clean

# binary absent: crate builds, tests excluded
cargo test --lib --no-run                                          # clean

# binary present: tests come back
cargo test --lib -- --list                                         # tests::test_transfer_sol: test