Fix test module path in riscv.rs to resolve correctly under Rust module rules

[CaiWeiran]

What this PR does

This PR fixes a build failure when running ./x test due to Rust being unable to locate the tests module declared in library/std_detect/src/detect/os/riscv.rs.
Rust's default module resolution expects mod tests; to resolve to os/tests.rs or os/tests/mod.rs. However, in this case, the tests are located in os/riscv/tests.rs.
To correct this, I explicitly set the path using:

#[cfg(test)]
#[path = "riscv/tests.rs"]
mod tests;

This allows the test module to be compiled correctly when testing, without needing to inline or move the file.

Why it's needed

Without this change, ./x test fails with:

error[E0583]: file not found for module `tests`

This fix enables successful test discovery and execution on riscv.rs.

Notes

• No functional logic changes were made.
• Only affects test compilation.

[rustbot]

r? @Mark-Simulacrum

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