ETR01SDK-544: Fix strict compilation target paths

[andreondra]

Description

In this PR I fix paths of files on which we apply strict compilation flags.

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• 🐛 Bug fix
• ✨ New feature
• 🧹 Code cleanup or refactoring
• 📝 Documentation update
• 🔧 Build system or toolchain update
• 🔒 Security improvement
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[Copilot]

Pull request overview

This PR fixes the paths used when applying strict compilation flags in CMake configuration files. The issue was that some test directories were referencing hardcoded file paths instead of using the already-defined ${SOURCES} variable, and one path was incorrect (referencing a non-existent src/main.c file).

Changes:

• Updated four CMakeLists.txt/common.cmake files to use ${SOURCES} variable instead of hardcoded paths when setting strict compilation properties
• Fixed incorrect path reference in usb_devkit that pointed to non-existent src/main.c

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
tests/functional/model/CMakeLists.txt	Changed strict compilation flags to use ${SOURCES} instead of hardcoded main.c path
tests/functional/linux/usb_devkit/CMakeLists.txt	Fixed incorrect src/main.c reference and changed to use ${SOURCES} variable
tests/functional/linux/spi/CMakeLists.txt	Changed strict compilation flags to use ${SOURCES} instead of hardcoded main.c path
tests/functional/esp32/common.cmake	Changed strict compilation flags to use ${SOURCES} instead of hardcoded main.c path

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