tool_operate: don't truncate the etag save file by default

This fixes a regression of 75d79a4486b279100209ddf8c7fdb12955fb66e9. The
code in tool-operate truncated the etag save file, und

tool_operate: don't truncate the etag save file by default

This fixes a regression of 75d79a4486b279100209ddf8c7fdb12955fb66e9. The
code in tool-operate truncated the etag save file, under the assumption
that the file would be written with a new etag value. However since
75d79a4486b279100209ddf8c7fdb12955fb66e9 that might not be the case
anymore and could result in the file being truncated when --etag-compare
and --etag-save was used and that the etag value matched with what the
server responded. Instead the truncation should not be done when a new
etag value should be written.

Test 3204 was added to verify that the file with the etag value doesn't
change the contents when used by --etag-compare and --etage-save and
that value matches with what the server returns on a non 2xx response.

Closes #13432

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