all: fix t.Skipf formats

Found by upcoming cmd/vet change.

Change-Id: I7a8264a304b2a4f26f3bd418c1b28cc849889c9b
Reviewed-on: https://go-review.googlesource.com/83835
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/go_test.go

@@ -224,7 +224,7 @@ func testgo(t *testing.T) *testgoData {
 	testenv.MustHaveGoBuild(t)
 
 	if skipExternal {
-		t.Skip("skipping external tests on %s/%s", runtime.GOOS, runtime.GOARCH)
+		t.Skipf("skipping external tests on %s/%s", runtime.GOOS, runtime.GOARCH)
 	}
 
 	return &testgoData{t: t}
@@ -2284,7 +2284,7 @@ func TestSymlinkWarning(t *testing.T) {
 	tg.tempDir("yy/zz")
 	tg.tempFile("yy/zz/zz.go", "package zz\n")
 	if err := os.Symlink(tg.path("yy"), tg.path("src/example/xx/yy")); err != nil {
-		t.Skip("symlink failed: %v", err)
+		t.Skipf("symlink failed: %v", err)
 	}
 	tg.run("list", "example/xx/z...")
 	tg.grepStdoutNot(".", "list should not have matched anything")

src/cmd/vendor/golang.org/x/arch/arm64/arm64asm/objdumpext_test.go

@@ -35,7 +35,7 @@ func testObjdumpArch(t *testing.T, generate func(func([]byte)), arch Mode) {
 func checkObjdumpAarch64(t *testing.T) {
 	out, err := exec.Command(objdumpPath, "-i").Output()
 	if err != nil {
-		t.Skip("cannot run objdump: %v\n%s", err, out)
+		t.Skipf("cannot run objdump: %v\n%s", err, out)
 	}
 	if !strings.Contains(string(out), "aarch64") {
 		t.Skip("objdump does not have aarch64 support")

src/runtime/crash_cgo_test.go

@@ -472,7 +472,7 @@ func TestWindowsStackMemoryCgo(t *testing.T) {
 func TestSigStackSwapping(t *testing.T) {
 	switch runtime.GOOS {
 	case "plan9", "windows":
-		t.Skip("no sigaltstack on %s", runtime.GOOS)
+		t.Skipf("no sigaltstack on %s", runtime.GOOS)
 	}
 	t.Parallel()
 	got := runTestProg(t, "testprogcgo", "SigStack")