Fix sqllogictests test running compatibility (ignore --test-threads) (apache#16694)

* Fix test running compatibility

sqllogictests.rs already has various parts of it's Options struct that
provide compatibility with the standard test running options.

Extend this to include the standard `--test-threads` option, so that
running for example `cargo test -- --test-threads 1` in the project root
will succeed.  Previously it complained about the unknown option.

This can be useful on a machine with a large number of cores which causes
many tests to build & run in parallel, which in my case caused a crash
due to exhausing the system RAM.

* Make test-threads optional

Make test-threads optional, so that the tests also pass when this is not
passed in. 🤦

Author: mjgarton

datafusion/sqllogictest/bin/sqllogictests.rs

@@ -689,6 +689,12 @@ struct Options {
         help = "IGNORED (for compatibility with built-in rust test runner)"
     )]
     nocapture: bool,
+
+    #[clap(
+        long,
+        help = "IGNORED (for compatibility with built-in rust test runner)"
+    )]
+    test_threads: Option<usize>,
 }
 
 impl Options {