[Feature]: Increase testing code coverage

### What would you like to see added to HDMF?

- @oruebel will look at the backends module
- @stephprince will look at the common module
- @rly will look at the build and spec modules

And then we will re-evaluate.

### What solution would you like?

Increase coverage to 95%

### Do you have any interest in helping implement the feature?

Yes.