testscript: stable error output [feature request]

[jpluscplusm]

This is a feature request either:

• for a CLI flag to enable the behaviour described below, or
• for the default behaviour of the testscript command to be updated to match this behaviour
  • with an optional CLI flag to opt back into the current behaviour.

Feature Request

Please consider adding a flag which causes the output from testscript in a failing context to be stable.

Rationale

Currently, both the stderr and stdout of a failing invocation include an unstable testscript[0-9]{10} path component tracking where the txtar archive was unpacked on the filesystem:

$ testscript test.txtar 
> ! exec true
FAIL: /tmp/testscript3999957216/test.txtar/script.txtar:1: unexpected command success
error running test.txtar in /tmp/testscript3999957216/test.txtar
$ testscript test.txtar 
> ! exec true
FAIL: /tmp/testscript1000854673/test.txtar/script.txtar:1: unexpected command success
error running test.txtar in /tmp/testscript1000854673/test.txtar

Its inclusion means that any automation that tracks/matches/enforces the output from such a failing test needs to special case recognising that path component, just in order to ignore it.

I don't believe that the significant majority of users running testscript will need to know this temporary prefix / path; and in the small minority of cases where it's important, then those situations have the -work flag available ("... prints the temporary work directory path before running each script"). I believe it would be harmless to drop the prefix when reporting an error, with an optional flag causing the command to revert to the current, unstable, output.