Fix compression tests in QEMU cross-compilation environments (#3248)

* tests: fix cmd_exists for QEMU environments

QEMU user-mode has a bug where posix_spawn returns success even when
the command doesn't exist. The child exits with 127, but the parent
thinks it succeeded.

Change cmd_exists to check if the command actually ran successfully
(exit code 0), not just if spawn returned Ok.

This fixes compression tests on riscv64 and other QEMU-emulated
architectures.

Ref https://github.com/rust-lang/rust/issues/90825

* tests: remove riscv64 skip for compression tests

Remove the cfg guards that disabled lz4, brotli, and zstd tests on
riscv64. These now work with the QEMU fix.

tests/misc.rs

@@ -922,8 +922,6 @@ be, to a very large extent, the result of luck. Sherlock Holmes
     eqnice!(expected, cmd.stdout());
 });
 
-// lz4 decompression tool doesn't work under RISC-V QEMU emulation in CI
-#[cfg(not(target_arch = "riscv64"))]
 rgtest!(compressed_lz4, |dir: Dir, mut cmd: TestCommand| {
     if !cmd_exists("lz4") {
         return;
@@ -954,8 +952,6 @@ be, to a very large extent, the result of luck. Sherlock Holmes
     eqnice!(expected, cmd.stdout());
 });
 
-// brotli decompression tool doesn't work under RISC-V QEMU emulation in CI
-#[cfg(not(target_arch = "riscv64"))]
 rgtest!(compressed_brotli, |dir: Dir, mut cmd: TestCommand| {
     if !cmd_exists("brotli") {
         return;
@@ -971,8 +967,6 @@ be, to a very large extent, the result of luck. Sherlock Holmes
     eqnice!(expected, cmd.stdout());
 });
 
-// zstd decompression tool doesn't work under RISC-V QEMU emulation in CI
-#[cfg(not(target_arch = "riscv64"))]
 rgtest!(compressed_zstd, |dir: Dir, mut cmd: TestCommand| {
     if !cmd_exists("zstd") {
         return;

tests/util.rs

@@ -45,7 +45,10 @@ pub fn sort_lines(lines: &str) -> String {
 /// Returns true if and only if the given program can be successfully executed
 /// with a `--help` flag.
 pub fn cmd_exists(program: &str) -> bool {
-    Command::new(program).arg("--help").output().is_ok()
+    match Command::new(program).arg("--help").output() {
+        Ok(output) => output.status.success(),
+        Err(_) => false,
+    }
 }
 
 /// Dir represents a directory in which tests should be run.