Respect PARQUET_TEST_DATA in variant_interop test (#7747)

# Which issue does this PR close?

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enhancements and this helps us generate change logs for our releases.
You can link an issue to this PR using the GitHub syntax.

- closes #7746

# Rationale for this change

The parquet-variant tests fail when run as part of
`verify-release-candidate.sh` due to the `parquet-testing` directory
being checked out in a different location

# What changes are included in this PR?

Update the test to look at the `PARQUET_TEST_DATA` environment variable
as well

# How are these changes tested?

I tested this manually:
```shell
# note this is a different name than the submodule:
git clone https://github.com/apache/parquet-testing.git parquet-testing-data
export PARQUET_TEST_DATA=$PWD/parquet-testing-data/data
# checkout my fork
git clone https://github.com/alamb/arrow-rs.git
cd arrow-rs
# This fails on main
git checkout main
cargo test -p parquet-variant
# PASSES on branch with fix
git checkout alamb/fix_variant_tests
cargo test -p parquet-variant
```

# Are there any user-facing changes?
No this is a test only change

Author: alamb

parquet-variant/tests/variant_interop.rs

@@ -18,21 +18,65 @@
 //! End-to-end check: (almost) every sample from apache/parquet-testing/variant
 //! can be parsed into our `Variant`.
 
-// NOTE: We keep this file separate rather than a test mod inside variant.rs because it should be
-// moved to the test folder later
-use std::fs;
 use std::path::{Path, PathBuf};
+use std::{env, fs};
 
 use chrono::NaiveDate;
 use parquet_variant::{
     ShortString, Variant, VariantBuilder, VariantDecimal16, VariantDecimal4, VariantDecimal8,
 };
 
+/// Returns a directory path for the parquet variant test data.
+///
+/// The data lives in the `parquet-testing` git repository:
+/// <https://github.com/apache/parquet-testing>
+///
+/// Normally this is checked out as a git submodule in the root of the `arrow-rs` repository,
+/// so the relative path is
+/// * `CARGO_MANIFEST_DIR/../parquet-testing/variant`.
+///
+/// However, the user can override this by setting the environment variable `PARQUET_TEST_DATA`
+/// to point to a different directory (as is done by the `verify-release-candidate.sh` script).
+///
+/// In this case, the environment variable `PARQUET_TEST_DATA` is expected to point to a directory
+/// `parquet-testing/data`, so the relative path to the `variant` subdirectory is
+/// * `PARQUET_TEST_DATA/../variant`.
 fn cases_dir() -> PathBuf {
-    Path::new(env!("CARGO_MANIFEST_DIR"))
+    // which we expect to point at "../parquet-testing/data"
+    let env_name = "PARQUET_TEST_DATA";
+    if let Ok(dir) = env::var(env_name) {
+        let trimmed = dir.trim();
+        if !trimmed.is_empty() {
+            let pb = PathBuf::from(trimmed).join("..").join("variant");
+            if pb.is_dir() {
+                return pb;
+            } else {
+                panic!(
+                    "Can't find variant data at `{pb:?}`. Used value of env `{env_name}`../variant ",
+                )
+            }
+        }
+    }
+
+    // PARQUET_TEST_DATA is undefined or its value is trimmed to empty, let's try default dir.
+
+    // env "CARGO_MANIFEST_DIR" is "the directory containing the manifest of your package",
+    // set by `cargo run` or `cargo test`, see:
+    // https://doc.rust-lang.org/cargo/reference/environment-variables.html
+    let pb = Path::new(env!("CARGO_MANIFEST_DIR"))
         .join("..")
         .join("parquet-testing")
-        .join("variant")
+        .join("variant");
+
+    if pb.is_dir() {
+        pb
+    } else {
+        panic!(
+            "env `{env_name}` is undefined or has empty value, and \
+             `CARGO_MANIFEST_DIR/../parquet-testing/variant` is not a directory: `{pb:?}`\n\
+             HINT: try running `git submodule update --init`",
+        )
+    }
 }
 
 struct Case {