Add .gitattributes, exclude tests from archive

[iamvar]

Description

Add .gitattributes, exclude tests from archive

Motivation and Context

less release size

Types of changes

• [x] Technical debt

[CLAassistant]

All committers have signed the CLA.

[phil-davis]

The official release tarballs are built from Makefile and other scripts that explicitly select and exclude files that are needed or not. So the official tarballs will not respect .gitattributes anyway.

Do we want to add a .gitattributes ?

@micbar or anyone - opinions please.