clusterd-test-driver: ignore lowering tests under miri (#37149)

### Motivation

The `mz-clusterd-test-driver` crate (added in #37008) is not in the miri
exclude list, so its unit tests run under nightly miri.
Every `dataflow::tests::*` test drives MIR-to-LIR lowering, which
reaches `mz_ore::stack::maybe_grow` -> `stacker` ->
`psm::stack_pointer`, a foreign function miri cannot call.
The tests therefore abort under miri (`unsupported operation: can't call
foreign function rust_psm_stack_pointer on OS linux`).

### Description

Mark the six lowering tests `#[cfg_attr(miri, ignore)]`, matching the
existing convention in `mz-expr` (e.g. `src/expr/src/visit.rs`,
`scalar.rs`, `relation/join_input_mapper.rs`), rather than excluding the
crate or shimming `mz_ore::stack`.

### Verification

`CARGO_TARGET_DIR=$PWD/miri-target MIRIFLAGS="-Zmiri-disable-isolation
-Zmiri-strict-provenance" cargo +nightly miri nextest run -p
mz-clusterd-test-driver dataflow::tests` skips them (exit 0); a normal
`cargo nextest run` still runs all of them.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: antiguru

src/clusterd-test-driver/src/dataflow.rs

@@ -646,6 +646,7 @@ mod tests {
     /// source import, a single object building `Get(source) -> ArrangeBy(key)`,
     /// and a single index export over the source.
     #[mz_ore::test]
+    #[cfg_attr(miri, ignore)] // error: unsupported operation: can't call foreign function `rust_psm_stack_pointer` on OS `linux`
     fn index_dataflow_structure() {
         let desc = crate::data::sample_desc();
         let loc = PersistLocation {
@@ -719,6 +720,7 @@ mod tests {
     /// arrange must lower to two distinct objects, and the index export must
     /// reference the built object rather than the source.
     #[mz_ore::test]
+    #[cfg_attr(miri, ignore)] // error: unsupported operation: can't call foreign function `rust_psm_stack_pointer` on OS `linux`
     fn build_computed_object_lowers() {
         let desc = crate::data::sample_desc();
         let loc = PersistLocation {
@@ -766,6 +768,7 @@ mod tests {
     /// MIR optimizer first, which fills the implementation, so the same dataflow
     /// then lowers. This is exactly what the `optimize` flag buys.
     #[mz_ore::test]
+    #[cfg_attr(miri, ignore)] // error: unsupported operation: can't call foreign function `rust_psm_stack_pointer` on OS `linux`
     fn join_lowers_only_with_optimize() {
         let loc = PersistLocation {
             blob_uri: "mem://".parse().unwrap(),
@@ -817,6 +820,7 @@ mod tests {
     /// same built object (binding). Both exports reference that object; the index
     /// arranges it and the MV sink writes it to a target shard.
     #[mz_ore::test]
+    #[cfg_attr(miri, ignore)] // error: unsupported operation: can't call foreign function `rust_psm_stack_pointer` on OS `linux`
     fn index_and_mv_same_binding() {
         let desc = crate::data::sample_desc();
         let loc = PersistLocation {
@@ -881,6 +885,7 @@ mod tests {
     /// import and no source imports, is the proof the index information reached the
     /// optimizer.
     #[mz_ore::test]
+    #[cfg_attr(miri, ignore)] // error: unsupported operation: can't call foreign function `rust_psm_stack_pointer` on OS `linux`
     fn optimize_uses_imported_index() {
         let desc = crate::data::sample_desc();
         let on_type = ReprRelationType::from(desc.typ());
@@ -913,6 +918,7 @@ mod tests {
     /// A count-over-index dataflow imports the index (no storage source), builds
     /// the reduce and its arrange as two objects, and exports the count index.
     #[mz_ore::test]
+    #[cfg_attr(miri, ignore)] // error: unsupported operation: can't call foreign function `rust_psm_stack_pointer` on OS `linux`
     fn count_over_index_structure() {
         let desc = crate::data::sample_desc();
         let on_type = ReprRelationType::from(desc.typ());