read to the end of tarballs

[StefanKarpinski]

This includes two changes to fix https://github.com/JuliaIO/ArgTools.jl/issues/20:

• Tar.list now reads to the end of the tarball instead of stopping reading when it hits a zero block
• test sets that expect errors now explicitly read to the end of the tarball inside of the @arg_test block

There's also an unrelated change to make_test_dir that bit me while making this change.

[codecov[bot]]

Codecov Report

Merging #115 (e3a0a3a) into master (fdeec31) will decrease coverage by 0.00%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #115      +/-   ##
==========================================
- Coverage   96.05%   96.05%   -0.01%     
==========================================
  Files           4        4              
  Lines         685      684       -1     
==========================================
- Hits          658      657       -1     
  Misses         27       27              

Impacted Files	Coverage Δ
src/extract.jl	96.58% <100.00%> (-0.01%)	:arrow_down:

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