Move variant interop test to Rust integration test (#7602)

# Which issue does this PR close?
- Part of https://github.com/apache/arrow-rs/issues/6736

# Rationale for this change

Rust integration tests (in `parquet-variant/tests`) are compiled as a
external program would be compiled and thus can only use the exposed
API. This helps verify that the crate is usable

# What changes are included in this PR?
1. Move the tests that read/write variant values into `variant_interop`
test (`cargo test --test variant_interop`)
2. Publically expose `pub` structures


# Are there any user-facing changes?
There are now pub APIs in the parquet-variant crate

parquet-variant/src/lib.rs

@@ -30,12 +30,9 @@
 // TODO: dead code removal
 #[allow(dead_code)]
 mod decoder;
-// TODO: dead code removal
-#[allow(dead_code)]
 mod variant;
 // TODO: dead code removal
 #[allow(dead_code)]
 mod utils;
 
-#[cfg(test)]
-mod test_variant;
+pub use variant::*;

parquet-variant/src/variant.rs

@@ -88,7 +88,7 @@ impl OffsetSizeBytes {
 }
 
 #[derive(Clone, Debug, Copy, PartialEq)]
-pub(crate) struct VariantMetadataHeader {
+pub struct VariantMetadataHeader {
     version: u8,
     is_sorted: bool,
     /// Note: This is `offset_size_minus_one` + 1
@@ -323,10 +323,16 @@ pub struct VariantArray<'m, 'v> {
 }
 
 impl<'m, 'v> VariantArray<'m, 'v> {
+    /// Return the length of this array
     pub fn len(&self) -> usize {
         todo!()
     }
 
+    /// Is the array of zero length
+    pub fn is_empty(&self) -> bool {
+        self.len() == 0
+    }
+
     pub fn values(&self) -> Result<impl Iterator<Item = Variant<'m, 'v>>, ArrowError> {
         todo!();
         #[allow(unreachable_code)] // Just to infer the return type

parquet-variant/tests/variant_interop.rs

@@ -23,8 +23,8 @@
 use std::fs;
 use std::path::{Path, PathBuf};
 
-use crate::variant::{Variant, VariantMetadata};
 use arrow_schema::ArrowError;
+use parquet_variant::{Variant, VariantMetadata};
 
 fn cases_dir() -> PathBuf {
     Path::new(env!("CARGO_MANIFEST_DIR"))
@@ -42,15 +42,14 @@ fn load_case(name: &str) -> Result<(Vec<u8>, Vec<u8>), ArrowError> {
 
 fn get_primitive_cases() -> Vec<(&'static str, Variant<'static, 'static>)> {
     vec![
-    ("primitive_boolean_false", Variant::BooleanFalse),
-    ("primitive_boolean_true", Variant::BooleanTrue),
-    ("primitive_int8", Variant::Int8(42)),
-    // Using the From<String> trait
-    ("primitive_string", Variant::from("This string is longer than 64 bytes and therefore does not fit in a short_string and it also includes several non ascii characters such as 🐢, 💖, ♥\u{fe0f}, 🎣 and 🤦!!")),
-    // Using the From<String> trait
-    ("short_string", Variant::from("Less than 64 bytes (❤\u{fe0f} with utf8)")), 
-    // TODO Reenable when https://github.com/apache/parquet-testing/issues/81 is fixed
-    // ("primitive_null", Variant::Null),
+        ("primitive_null", Variant::Null),
+        ("primitive_boolean_false", Variant::BooleanFalse),
+        ("primitive_boolean_true", Variant::BooleanTrue),
+        ("primitive_int8", Variant::Int8(42)),
+        // Using the From<String> trait
+        ("primitive_string", Variant::from("This string is longer than 64 bytes and therefore does not fit in a short_string and it also includes several non ascii characters such as 🐢, 💖, ♥\u{fe0f}, 🎣 and 🤦!!")),
+        // Using the From<String> trait
+        ("short_string", Variant::from("Less than 64 bytes (❤\u{fe0f} with utf8)")), 
     ]
 }