[OTel] unittest coverage improvement

[CagriYonca]

I worked on the modules below, increased existing unittests and created non-existing:

• src/instana/collector/base.py (100% coverage)
• src/instana/collector/helpers/runtime.py (97%)
• src/instana/collector/host.py (96%)
• src/instana/collector/utils.py (100%)
• src/instana/propagators/base_propagator.py (96%)
• src/instana/sampling.py (100%)
• src/instana/util/traceutils.py (100%)