fix broken truncated tarball tests

[StefanKarpinski]

In #107 the tests for throwing EOFError when a tarball is truncated were made significantly more systematic, which revealed that we don't always when we should. This should be fixed. I merged that PR with broken tests because not throwing errors for a truncated tarball isn't that big an issue and I wanted to get on with other things, but this should be fixed at some point. Note that the test-set for a truncated header is disabled for Julia 1.3; it's unclear whether these tests were failing due to a Julia bug that was fixed or not. It would be good to understand that issue as well.