fuse/test: TestFopenKeepCache: denoise

We have many calls to ioutil.WriteFile and ioutil.ReadFile + checking
for error. Move those calls into utility functions which call t.Fatal if
they see any error. No need to use additional error prefix as both
ioutil.WriteFile and ReadFile produce os.PathError which always has
opertion and path on which it was performed.

fuse/test/cache_test.go

@@ -74,27 +74,31 @@ func TestFopenKeepCache(t *testing.T) {
 	wd, pathfs, clean := setupCacheTest(t)
 	defer clean()
 
-	before := "before"
-	after := "after"
-	if err := ioutil.WriteFile(wd+"/orig/file.txt", []byte(before), 0644); err != nil {
-		t.Fatalf("WriteFile failed: %v", err)
+	// x{read,write}File reads/writes file@path and fail on error
+	xreadFile := func(path string) string {
+		data, err := ioutil.ReadFile(path)
+		if err != nil {
+			t.Fatal(err)
+		}
+		return string(data)
 	}
-
-	c, err := ioutil.ReadFile(wd + "/mnt/file.txt")
-	if err != nil {
-		t.Fatalf("ReadFile: %v", err)
-	} else if string(c) != before {
-		t.Fatalf("ReadFile: got %q, want %q", c, before)
+	xwriteFile := func(path, data string) {
+		if err := ioutil.WriteFile(path, []byte(data), 0644); err != nil {
+			t.Fatal(err)
+		}
 	}
 
-	if err := ioutil.WriteFile(wd+"/orig/file.txt", []byte(after), 0644); err != nil {
-		t.Fatalf("WriteFile: %v", err)
+	before := "before"
+	after := "after"
+	xwriteFile(wd+"/orig/file.txt", before)
+	c := xreadFile(wd + "/mnt/file.txt")
+	if c != before {
+		t.Fatalf("ReadFile: got %q, want %q", c, before)
 	}
 
-	c, err = ioutil.ReadFile(wd + "/mnt/file.txt")
-	if err != nil {
-		t.Fatalf("ReadFile: %v", err)
-	} else if string(c) != before {
+	xwriteFile(wd+"/orig/file.txt", after)
+	c = xreadFile(wd + "/mnt/file.txt")
+	if c != before {
 		t.Fatalf("ReadFile: got %q, want cached %q", c, before)
 	}
 
@@ -107,10 +111,8 @@ func TestFopenKeepCache(t *testing.T) {
 		t.Errorf("EntryNotify: %v", code)
 	}
 
-	c, err = ioutil.ReadFile(wd + "/mnt/file.txt")
-	if err != nil {
-		t.Fatalf("ReadFile: %v", err)
-	} else if string(c) != after {
+	c = xreadFile(wd + "/mnt/file.txt")
+	if c != after {
 		t.Fatalf("ReadFile: got %q after notify, want %q", c, after)
 	}
 }