retain output when expunging from multiple caches

[christofchen]

Short description

append to the output buffer in expungeByName (instead of overwriting)

Checklist

I have:

• [X ] read the CONTRIBUTING.md document
• [ X] compiled this code
• [ X] tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)
• [X ] checked that this code was merged to master

This fix addresses #12075

[rgacogne]

I was a bit worried about g_outputBuffer potentially still having some left-over data from the previous command, but it looks like we are careful to clear it between lines, so it should be OK.