reduce unnecessary std imports in tests

Since we have to recompile all dependencies, this reduces a substantial
amount of work, reducing 'go test -short' time from ~16s to ~12s on my
laptop.

testdata/scripts/asm.txt

@@ -1,13 +1,13 @@
 garble build
 exec ./main
-cmp stdout main.stdout
+cmp stderr main.stderr
 binsubstr main$exe 'privateAdd' 'PublicAdd'
 
 [short] stop # no need to verify this with -short
 
 go build
 exec ./main
-cmp stdout main.stdout
+cmp stderr main.stderr
 
 -- go.mod --
 module test/main
@@ -15,16 +15,14 @@ module test/main
 package main
 
 import (
-	"fmt"
-
 	"test/main/imported"
 )
 
 func privateAdd(x, y int64) int64
 
 func main() {
-	fmt.Println(privateAdd(1, 2))
-	fmt.Println(imported.PublicAdd(3, 4))
+	println(privateAdd(1, 2))
+	println(imported.PublicAdd(3, 4))
 }
 -- main.s --
 TEXT ·privateAdd(SB),$0-24
@@ -44,6 +42,6 @@ TEXT ·PublicAdd(SB),$0-24
 	ADDQ BP, BX
 	MOVQ BX, ret+16(FP)
 	RET
--- main.stdout --
+-- main.stderr --
 3
 7

testdata/scripts/cgo.txt

@@ -1,13 +1,13 @@
 garble build
 exec ./main
-cmp stdout main.stdout
+cmp stderr main.stderr
 binsubstr main$exe 'privateAdd'
 
 [short] stop # no need to verify this with -short
 
 go build
 exec ./main
-cmp stdout main.stdout
+cmp stderr main.stderr
 
 -- go.mod --
 module test/main
@@ -21,10 +21,8 @@ static int privateAdd(int a, int b) {
 */
 import "C"
 
-import "fmt"
-
 func main() {
-	fmt.Println(C.privateAdd(C.int(1), C.int(2)))
+	println(C.privateAdd(C.int(1), C.int(2)))
 }
--- main.stdout --
+-- main.stderr --
 3

testdata/scripts/ldflags.txt

@@ -1,13 +1,13 @@
 garble build -ldflags='-X=main.unexportedVersion=v1.0.0 -X=test/main/imported.ExportedVar=replaced'
 exec ./main
-cmp stdout main.stdout
+cmp stderr main.stderr
 ! binsubstr main$exe 'unexportedVersion'
 
 [short] stop # no need to verify this with -short
 
 exec go build -ldflags='-X=main.unexportedVersion=v1.0.0 -X=test/main/imported.ExportedVar=replaced'
 exec ./main
-cmp stdout main.stdout
+cmp stderr main.stderr
 binsubstr main$exe 'unexportedVersion'
 
 -- go.mod --
@@ -16,21 +16,19 @@ module test/main
 package main
 
 import (
-	"fmt"
-
 	"test/main/imported"
 )
 
 var unexportedVersion = "unknown"
 
 func main() {
-	fmt.Println("version:", unexportedVersion)
-	fmt.Println("var:", imported.ExportedVar)
+	println("version:", unexportedVersion)
+	println("var:", imported.ExportedVar)
 }
 -- imported/imported.go --
 package imported
 
 var ExportedVar = "original"
--- main.stdout --
+-- main.stderr --
 version: v1.0.0
 var: replaced

testdata/scripts/modinfo.txt

@@ -1,13 +1,13 @@
 garble build
 exec ./main
-cmp stdout main.stdout
+cmp stderr main.stderr
 ! binsubstr main$exe '(devel)'
 
 [short] stop # no need to verify this with -short
 
 exec go build
 exec ./main
-cmp stdout main.stdout-orig
+cmp stderr main.stderr-orig
 binsubstr main$exe '(devel)'
 
 -- go.mod --
@@ -15,15 +15,16 @@ module test/main
 -- main.go --
 package main
 
-import (
-	"fmt"
-	"runtime/debug"
-)
+import "runtime/debug"
 
 func main() {
-	fmt.Println(debug.ReadBuildInfo())
+	if info, ok := debug.ReadBuildInfo(); ok {
+		println("version", info.Main.Version)
+	} else {
+		println("no version")
+	}
 }
--- main.stdout-orig --
-&{test/main {test/main (devel)  <nil>} []} true
--- main.stdout --
-<nil> false
+-- main.stderr-orig --
+version (devel)
+-- main.stderr --
+no version

testdata/scripts/plugin.txt

@@ -7,7 +7,7 @@ binsubstr plugin.so 'PublicVar' 'PublicFunc'
 # Note that we need -trimpath; see the caveat section in the README.
 go build -trimpath
 exec ./main
-cmp stdout main.stdout
+cmp stderr main.stderr
 binsubstr main$exe 'PublicVar' 'PublicFunc'
 ! binsubstr plugin.so 'privateFunc'
 
@@ -17,18 +17,16 @@ go build -buildmode=plugin ./plugin
 binsubstr plugin.so 'PublicVar' 'PublicFunc' 'privateFunc'
 go build
 exec ./main
-cmp stdout main.stdout
+cmp stderr main.stderr
 
 -- go.mod --
 module test/main
 -- plugin/main.go --
 package main
 
-import "fmt"
-
 var PublicVar int
 
-func privateFunc(n int) { fmt.Printf("Hello, number %d\n", n) }
+func privateFunc(n int) { println("Hello, number", n) }
 
 func PublicFunc() { privateFunc(PublicVar) }
 -- main.go --
@@ -52,5 +50,5 @@ func main() {
 	*v.(*int) = 7
 	f.(func())()
 }
--- main.stdout --
+-- main.stderr --
 Hello, number 7