cmd/callgraph: add test of -algo=vta
cmd/callgraph: add test of -algo=vta

This should have been included in
https://go-review.googlesource.com/c/tools/+/410138

Change-Id: Ic8a4f1b5dbcdb320fb872b24484b830dcb56fd41
Reviewed-on: https://go-review.googlesource.com/c/tools/+/410123
Run-TryBot: Alan Donovan <7069e9552f9d110b0a0f326b96f22f63fb99dfc1@google.com>
Reviewed-by: Tim King <ee8a1d1efb05d8171fc0140ced15d455e83941c5@google.com>
gopls-CI: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Zvonimir Pavlinovic <490af24f3def1258d2f7e9b10b20e341f6c68320@google.com>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools