Update README.linux testing information

[candrews]

Update testing instructions to align with changes made during the cmake migration.

Description

The instructions in README.linux regarding testing are incorrect. This change fixes that issue.

Motivation and Context

The documentation is wrong - enough said :-)

How Has This Been Tested?

Screenshots (if appropriate):

Types of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the Code guidelines of this project
• My change requires a change to the documentation, either Doxygen or wiki
• I have updated the documentation accordingly
• I have read the CONTRIBUTING document
• I have added tests to cover my change
• All new and existing tests passed

[hudokkow]

Thanks!