Increase determinism in DiskLruCache tests.

This kills a flaky test by dependency injecting the executor and using a test version as a stronger signal.
Has a few users...
```
$ grep -r 'writeUtf8(Long.toString' .
[redacted]:448:    sink.writeUtf8(Long.toString(value));
[redacted]/JsonWriter.java:448:    sink.writeUtf8(Long.toString(value));
./square/okhttp/okhttp/src/main/java/com/squareup/okhttp/internal/DiskLruCache.java:970:        writer.writeByte(' ').writeUtf8(Long.toString(length));
./square/okhttp/okhttp/src/main/java/com/squareup/okhttp/MultipartBuilder.java:228:          .writeUtf8(Long.toString(contentLength))
[redacted]:267:            sink.writeUtf8("Content-Length: ").writeUtf8(Long.toString(contentLength)).write(CRLF);

$ grep -r 'writeUtf8(Integer.toString' .
./square/okhttp/okhttp/src/main/java/com/squareup/okhttp/Cache.java:484:      sink.writeUtf8(Integer.toString(varyHeaders.size()));
./square/okhttp/okhttp/src/main/java/com/squareup/okhttp/Cache.java:495:      sink.writeUtf8(Integer.toString(responseHeaders.size()));
./square/okhttp/okhttp/src/main/java/com/squareup/okhttp/Cache.java:539:        sink.writeUtf8(Integer.toString(certificates.size()));
./square/okhttp/okhttp/src/main/java/com/squareup/okhttp/internal/DiskLruCache.java:360:      writer.writeUtf8(Integer.toString(appVersion)).writeByte('\n');
./square/okhttp/okhttp/src/main/java/com/squareup/okhttp/internal/DiskLruCache.java:361:      writer.writeUtf8(Integer.toString(valueCount)).writeByte('\n');
[redacted]:254:            .writeUtf8(Integer.toString(response.code()))
```