testscript: mention when -continue is used

[jpluscplusm]

Feature request

Add some mention of the fact that -continue is in effect, in the testscript command's output to stdout.

Rationale

When I create a txtar repro and need to capture its actual ("not as expected") output, I can invoke testscript foo.txt | wl-copy (where wl-copy is a command that places its input into my copy/paste buffer) and paste the output directly into the issue I'm opening.

However, whenever the narrative flow in my repro requires the use of -continue, I need to:

• invoke testscript -continue foo.txt | wl-copy
• paste the output into the body of the issue I'm opening
• switch back to the terminal
• copy the first element of the invocation
• paste the invoked command as a preamble to the testscript output
• fix up the command by adding a $ prefix, indicating it's a command to the reader.

In other words: to nicely inform the reader that they're about to see a testscript -continue invocation, I need to manually add it before the record of the command's output as I can't rely on the command's stdout to indicate that the flag is present.

Sure, I could assume that the reader should be able to infer the presence of -continue from the presence of multiple FAIL:-prefixed lines, but I don't think I'm the only person to feel an obligation to make it clearer. I've definitely seen examples of other folks manually adding the testscript -continue ... prefix to repros in issues in https://github.com/cue-lang/cue.