[dev.fuzz] internal/fuzz: add stub for coverage

This change only includes a stub for the function
which will hook into the runtime to expose
coverage instrumentation while we're fuzzing.

Previously, we discussed an exported API named
FuzzCoverage, but since this is within the
internal/fuzz package, simply naming it coverage
seems appropriate.

Change-Id: Iba3240e53e0c4c434e937aa9bb1711a44fec9975
Reviewed-on: https://go-review.googlesource.com/c/go/+/308191
Trust: Katie Hockman <katie@golang.org>
Run-TryBot: Katie Hockman <katie@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/internal/fuzz/coverage.go

@@ -0,0 +1,11 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fuzz
+
+// coverage returns a []byte containing unique 8-bit counters for each edge of
+// the instrumented source code. This coverage data will only be generated if
+// `-d=libfuzzer` is set at build time. This can be used to understand the code
+// coverage of a test execution.
+func coverage() []byte { return nil }