cmake: Fix condition to disable plugin tests

[RytoEX]

Description

The CMake generator expression to enable plugin tests mistakenly used a string literal, which evaluated to 1, which enabled the plugin tests. Use variable notation to correctly test this value.

Motivation and Context

How Has This Been Tested?

Tested OS(s):

• Windows 11 Tested configured builds locally and checked the resulting value of PLUGIN_TESTS.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• [x] I have read the Contributing Guidelines.
• [x] All commit messages are properly formatted and commits squashed where appropriate.
• [x] My code is not on master or a release/* branch.
• [x] The code has been tested.
• [x] I have included updates to all appropriate documentation.