test-tool: show tool list on error

test-tool: show tool list on error

Before we switched to one big test-tool binary, if you
forgot the name of a tool, you could use tab-completion in
the shell to get a hint. But these days, all you get is:

  $ t/helper/test-tool approxidate
  fatal: There is no test named 'approxidate'

and you're stuck reading the source code to find it. Let's
print a list of the available tools in this case.

Signed-off-by: Jeff King <redacted>
Reviewed-by: Derrick Stolee <redacted>
Signed-off-by: Junio C Hamano <redacted>