[SPARK-15978][SQL] improve 'show tables' command related codes

## What changes were proposed in this pull request?

I've found some minor issues in "show tables" command:

1. In the `SessionCatalog.scala`, `listTables(db: String)` method will call `listTables(formatDatabaseName(db), "*")` to list all the tables for certain db, but in the method `listTables(db: String, pattern: String)`, this db name is formatted once more. So I think we should remove
`formatDatabaseName()` in the caller.

2. I suggest to add sort to listTables(db: String) in InMemoryCatalog.scala, just like listDatabases().

## How was this patch tested?

The existing test cases should cover it.

Author: bomeng <bmeng@us.ibm.com>

Closes #13695 from bomeng/SPARK-15978.