Limit Coverage Flags to Specific Target

[threeal]

This pull request introduces the following changes:

• Set coverage flags only to the my_fibonacci_test target.
• Rebuild source files of the my_fibonacci target instead of linking it to the my_fibonacci_test target. This changes prevent the my_fibonacci target from having coverage flags enabled.
• Modifies check coverage steps as follows:
  • Removes src/main.cpp file from exclude input, basically include all files but it's safe now because the coverage will only enabled to source files related in testing.
  • Set fail-under-line to be 100, make sure test always covers everything.

It closes #64.