…and-line

Snapshot errors
---------------

The constructor of `IntegSnapshotRunner` calls `loadManifest()`, which
on my computer happened to fail and stopped the entire test suite
because this error happened outside the `try/catch` block.

Move it inside the try/catch block, needed a bit of refactoring to
make sure we could still get at the test name.

`--from-file`
-------------

Instead of having `--from-file` require a JSON file with its own
structure, interpret it as a text file which gets treated exactly
the same as the `[TEST [..]]` arguments on the command line.

This still allows for the `--exclude` behavior by setting that flag
on the command-line.

Refactoring
-----------

Moved the logic around determining test names and directories into a
class (`IntegTest`) which is a convenience class on top of a static data
record (`IntegTestInfo`). We pass the data record to worker threads.

Print snapshot validations that are running for a long time (1 minute).
This helps diagnose what is stuck, if anything is. On my machine, it
was tests using Docker because there was some issue with it, and this
lost me a day.

Also change the test reporting formatting slightly, and catch errors
during actual test running.