testscript: add support for Go master aka 1.18

Fuzzing support brought a new parameter to MainStart,
as well as more methods on testDeps.
Use build tags to support those without breaking 1.16 nor 1.17.

While at it, bump CI to test the two latest Go versions.

Fixes #144.

.github/workflows/test.yml

@@ -11,7 +11,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        go-version: [1.15.x, 1.16.x]
+        go-version: [1.16.x, 1.17.x]
         os: [ubuntu-latest, macos-latest, windows-latest]
     runs-on: ${{ matrix.os }}
     steps:

go.mod

@@ -1,6 +1,6 @@
 module github.com/rogpeppe/go-internal
 
-go 1.15
+go 1.16
 
 require (
 	github.com/pkg/diff v0.0.0-20210226163009-20ebb0f2a09e

testscript/exe.go

@@ -15,7 +15,6 @@ import (
 	"path/filepath"
 	"runtime"
 	"strings"
-	"testing"
 )
 
 // TestingM is implemented by *testing.M. It's defined as an interface
@@ -228,7 +227,7 @@ func runCoverSubcommand(cprof string, mainf func() int) (exitCode int) {
 
 		// Run MainStart (recursively, but it we should be ok) with no tests
 		// so that it writes the coverage profile.
-		m := testing.MainStart(nopTestDeps{}, nil, nil, nil)
+		m := mainStart()
 		if code := m.Run(); code != 0 && exitCode == 0 {
 			exitCode = code
 		}

testscript/exe_go118.go

@@ -0,0 +1,50 @@
+//go:build go1.18
+// +build go1.18
+
+package testscript
+
+import (
+	"reflect"
+	"testing"
+	"time"
+)
+
+func mainStart() *testing.M {
+	return testing.MainStart(nopTestDeps{}, nil, nil, nil, nil)
+}
+
+// Note: corpusEntry is an anonymous struct type used by some method stubs.
+type corpusEntry = struct {
+	Parent     string
+	Name       string
+	Data       []byte
+	Values     []interface{}
+	Generation int
+	IsSeed     bool
+}
+
+// Note: CoordinateFuzzing was added in Go 1.18.
+func (nopTestDeps) CoordinateFuzzing(time.Duration, int64, time.Duration, int64, int, []corpusEntry, []reflect.Type, string, string) error {
+	return nil
+}
+
+// Note: RunFuzzWorker was added in Go 1.18.
+func (nopTestDeps) RunFuzzWorker(func(corpusEntry) error) error {
+	return nil
+}
+
+// Note: ReadCorpus was added in Go 1.18.
+func (nopTestDeps) ReadCorpus(string, []reflect.Type) ([]corpusEntry, error) {
+	return nil, nil
+}
+
+// Note: CheckCorpus was added in Go 1.18.
+func (nopTestDeps) CheckCorpus([]interface{}, []reflect.Type) error {
+	return nil
+}
+
+// Note: ResetCoverage was added in Go 1.18.
+func (nopTestDeps) ResetCoverage() {}
+
+// Note: SnapshotCoverage was added in Go 1.18.
+func (nopTestDeps) SnapshotCoverage() {}

testscript/exe_pre_go1.18.go

@@ -0,0 +1,12 @@
+//go:build !go1.18
+// +build !go1.18
+
+package testscript
+
+import (
+	"testing"
+)
+
+func mainStart() *testing.M {
+	return testing.MainStart(nopTestDeps{}, nil, nil, nil)
+}