x509roots/fallback: add init time benchmark

goos: linux
goarch: amd64
pkg: golang.org/x/crypto/x509roots/fallback
cpu: AMD Ryzen 5 4600G with Radeon Graphics
            │ /tmp/before │
            │   sec/op    │
InitTime-12   1.726m ± 0%

            │ /tmp/before  │
            │     B/op     │
InitTime-12   1.151Mi ± 0%

            │ /tmp/before │
            │  allocs/op  │
InitTime-12   11.35k ± 0%

For golang/go#73691

Change-Id: Ic932bd7835e50dd5c6adbdf684644afa49bddebc
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/676216
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Sean Liao <sean@liao.dev>
Auto-Submit: Sean Liao <sean@liao.dev>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

x509roots/fallback/fallback_test.go

@@ -0,0 +1,11 @@
+package fallback
+
+import "testing"
+
+// BenchmarkInitTime benchmarks the time it takes to parse all certificates
+// in this bundle, it corresponds to the init time of this package.
+func BenchmarkInitTime(b *testing.B) {
+	for range b.N {
+		newFallbackCertPool()
+	}
+}