[#139] test: Add test storage implementation

This aims to reduce the usage of chmod hackery to induce or simulate
OS-related failures.

Signed-off-by: Alejandro Lopez <a.lopez@yadro.com>

pkg/local_object_storage/writecache/options.go

@@ -1,6 +1,8 @@
 package writecache
 
 import (
+	"io/fs"
+	"os"
 	"time"
 
 	"git.frostfs.info/TrueCloudLab/frostfs-node/pkg/local_object_storage/blobstor"
@@ -55,6 +57,8 @@ type options struct {
 	noSync bool
 	// reportError is the function called when encountering disk errors in background workers.
 	reportError func(string, error)
+	// openFile is the function called internally by bbolt to open database files. Useful for hermetic testing.
+	openFile func(string, int, fs.FileMode) (*os.File, error)
 }
 
 // WithLogger sets logger.
@@ -152,3 +156,10 @@ func WithReportErrorFunc(f func(string, error)) Option {
 		o.reportError = f
 	}
 }
+
+// WithOpenFile sets the OpenFile function to use internally by bolt. Useful for hermetic testing.
+func WithOpenFile(f func(string, int, fs.FileMode) (*os.File, error)) Option {
+	return func(o *options) {
+		o.openFile = f
+	}
+}