gopls/internal/regtest/bench: add an oracle completion benchmark

The enormous dataintegration package in oracle demonstrates a clear
regression from [email protected] in completion CPU utilization during
autocompletion.

On my machine latency appears about the same, but on more
resource-constrained machines this may not be the case.

For golang/go#61207

Change-Id: I59631f34fe0d8d5d3329c9444d4e485840ad85ed
Reviewed-on: https://go-review.googlesource.com/c/tools/+/516678
TryBot-Result: Gopher Robot <[email protected]>
gopls-CI: kokoro <[email protected]>
Run-TryBot: Robert Findley <[email protected]>
Reviewed-by: Hyang-Ah Hana Kim <[email protected]>

Author: findleyr

Diff for: gopls/internal/regtest/bench/completion_test.go

@@ -180,6 +180,18 @@ func (kl *Kubelet) _() {
 `,
 			`kl\.()`,
 		},
+		{
+			"oracle",
+			"dataintegration/pivot2.go",
+			`
+package dataintegration
+
+func (p *Pivot) _() {
+	p.
+}
+`,
+			`p\.()`,
+		},
 	}
 
 	for _, test := range tests {