lastLocation testing

[jglick]

Pairs with jenkinsci/durable-task-plugin#187. I noticed a flake in ExecutorStepTest.contextualizeFreshFilePathAfterAgentReconnection[watching=true]: the test timed out, and the log included

WARNING: org.jenkinsci.plugins.durabletask.FileMonitoringTask$Watcher@… giving up on watching …/dumbo/workspace/demo@tmp/durable-…
java.lang.NumberFormatException: For input string: ""
	at java.base/java.lang.NumberFormatException.forInputString(NumberFormatException.java:65)
	at java.base/java.lang.Long.parseLong(Long.java:702)
	at java.base/java.lang.Long.parseLong(Long.java:817)
	at org.jenkinsci.plugins.durabletask.FileMonitoringTask$Watcher.run(FileMonitoringTask.java:622)

I do not have a clear hypothesis as to why lastLocation was empty; perhaps a half-written file could be to blame, in which case using AtomicFileWriter could help, but this seems dubious when the same process should have already finished writing the same file.