Compile tests only when required - Only compile tests when BUILD_TESTS flags is ON

[sirpipat]

Description

Added BUILD_TESTS flag to CMakeLists.txt. The default is set to OFF. To enable BUILD_TESTS flag, added -DBUILD_TESTS=ON to the command during the compilation.

Issue Number

Issue #63

Checklist

Please make sure to check developer documentation on specfem docs.

• [x] I ran the code through pre-commit to check style
• [ ] My code passes all the integration tests
• [ ] I have added sufficient unittests to test my changes
• [x] I have added/updated documentation for the changes I am proposing
• [x] I have updated CMakeLists to ensure my code builds
• [ ] My code builds across all platforms

[buildbot-princeton]

Can one of the admins verify this patch?

[buildbot-princeton]

Can one of the admins verify this patch?

[Rohit-Kakodkar]

ok to test