tests: attempt to gather code coverage info in the executable tests

[pkgw]

It turns out that our tests of the built tectonic executable weren't gathering code coverage information about the code paths exercised in those tests. I think that it's not so hard to hack in some support to gather that information, so let's give it a try.

CC @ralismark @Mrmaxmeier

xref: #636 assert-rs/assert_cmd#9

[codecov]

Codecov Report

Merging #638 into master will increase coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #638      +/-   ##
==========================================
+ Coverage   46.25%   46.32%   +0.07%     
==========================================
  Files         139      131       -8     
  Lines       60448    59079    -1369     
==========================================
- Hits        27958    27367     -591     
+ Misses      32490    31712     -778     

Impacted Files	Coverage Δ
tests/trip.rs
tests/util/mod.rs
tests/driver.rs
tests/formats.rs
tests/cached_itarbundle.rs
tests/bibtex.rs
tests/tex-outputs.rs
tectonic/dpx-pdfobj.c	46.13% <0.00%> (+0.12%)	⬆️
tectonic/dpx-t1_char.c	68.43% <0.00%> (+0.43%)	⬆️
tectonic/dpx-dpxcrypt.c	17.52% <0.00%> (+0.53%)	⬆️
... and 13 more

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