Add test for "show techsupport" utility command

[alpeshspatel]

Description

This is a test enhancement request per comment from @yxieca in the PR https://github.com/sonic-net/sonic-utilities/pull/2403

The issue is opened to add a unit test to verify "show techsupport" command operation

Describe the results you expected

• non-zero file size
• operation completed successfully (diskspace issue, failures did not result in command failure)
• the gzipped tarball was not corrupt
• additionally, confirm if required files were captured (syslog, config for e.g.) based on file names