Add GitHub Actions

[NicMcPhee]

That would be useful for both the students and the TAs. This should include:

• [x] GTest to make sure the tests still pass
• [x] valgrind to make sure there are no memory leaks
• [x] Make badges out of these if possible

There are GitHub Actions that include gtest now, so hopefully that won't be too big a deal.

It looks like we might need to install valgrind in the container that runs the tests. Apparently this will do the job:

sudo apt install -y valgrind

but takes rather a while to do the install.

[NicMcPhee]

This was done for (at least) the first two C labs, so we know how to bring it to this one as well. We just need to do it.