930f12f1

By: Tom Sydney Kerckhove <syd@cs-syd.eu>

Have assert-score symlink reports into an out dir on success

Add --out-dir to the assert-score subcommand.  When set and the
assertion passes, the driver creates the directory (if absent) and
symlinks report.txt and report.json from the report directory into
it.

assertMutationScore.nix's buildCommand shrinks to a single subcommand
invocation: no mkdir, no ln -s, no Bash glue at all.

Tests cover three runAssertScore paths: pass with --out-dir populates
the directory, fail aborts before symlinking, missing report.json
exits with code 2.

Timeline