Expose internal APIs for pluggable file systems.

We aren't yet ready to make FileSystem a public type, but I don't mind
making it _almost_ available via an internal API for those brave enough
to try that.

Also migrate all of our tests to use the in memory file system. It's simpler.

https://github.com/square/okhttp/issues/1459

okhttp-testing-support/src/main/java/com/squareup/okhttp/internal/io/InMemoryFileSystem.java

@@ -0,0 +1,75 @@
+/*
+ * Copyright (C) 2015 Square, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.squareup.okhttp.internal.io;
+
+import java.io.File;
+import java.io.FileNotFoundException;
+import java.io.IOException;
+import java.util.Iterator;
+import java.util.LinkedHashMap;
+import java.util.Map;
+import okio.Buffer;
+import okio.Sink;
+import okio.Source;
+
+/** A simple file system where all files are held in memory. Not safe for concurrent use. */
+public final class InMemoryFileSystem implements FileSystem {
+  private final Map<File, Buffer> files = new LinkedHashMap<>();
+
+  @Override public Source source(File file) throws FileNotFoundException {
+    Buffer result = files.get(file);
+    if (result == null) throw new FileNotFoundException();
+    return result.clone();
+  }
+
+  @Override public Sink sink(File file) throws FileNotFoundException {
+    Buffer result = new Buffer();
+    files.put(file, result);
+    return result;
+  }
+
+  @Override public Sink appendingSink(File file) throws FileNotFoundException {
+    Buffer result = files.get(file);
+    return result != null ? result : sink(file);
+  }
+
+  @Override public void delete(File file) throws IOException {
+    files.remove(file);
+  }
+
+  @Override public boolean exists(File file) throws IOException {
+    return files.containsKey(file);
+  }
+
+  @Override public long size(File file) {
+    Buffer buffer = files.get(file);
+    return buffer != null ? buffer.size() : 0L;
+  }
+
+  @Override public void rename(File from, File to) throws IOException {
+    Buffer buffer = files.remove(from);
+    if (buffer == null) throw new FileNotFoundException();
+    files.put(to, buffer);
+  }
+
+  @Override public void deleteContents(File directory) throws IOException {
+    String prefix = directory.toString() + "/";
+    for (Iterator<File> i = files.keySet().iterator(); i.hasNext(); ) {
+      File file = i.next();
+      if (file.toString().startsWith(prefix)) i.remove();
+    }
+  }
+}