RFR: CODETOOLS-7903091: Convert jtreg to use NIO [v2]

[Jonathan Gibbons]

> Please review a medium-big, conceptually simple but occasionally tricky update to jtreg, to use `java.nio.file.Path` instead of `java.io.File` where reasonable.
> 
> While most of the changes are formulaic and follow the obvious patterns, there were some noteworthy points:
> 
> * The most complicated class to change was `Locations`, which does the most work to derive new paths. In particular, the semantics of `new File(a, b)` do not always match `Path.of(a).resolve(b)` for corner cases like when `a` is`null` or empty, or when `b` is absolute.
> 
> * The NIO equivalent for some operations on `File` throw checked IO exceptions.  These methods were wrapped to to throw a custom unchecked IO exception.
> 
> * The NIO equivalent for creating a new file (`Path`) throws an unchecked `InvalidPathException` exception. The use sites are too numerous to handle individually.  The solution is to catch exceptions like these (and the ones described in the previous bullet) at strategic points where they can be handled or wrapped back into (existing) checked exceptions.
> 
> The development has been iterative, frequently running the jtreg internal tests locally (on a Mac) and occasionally building on a CI system (Linux).  We should do more exhaustive testing with JDK before this work ever gets promoted.

Jonathan Gibbons has updated the pull request incrementally with one additional commit since the last revision:

  Address review comments
  Add debug output for failing test

-------------

Changes:
  - all: https://git.openjdk.java.net/jtreg/pull/50/files
  - new: https://git.openjdk.java.net/jtreg/pull/50/files/3d4f7dfd..45328538

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jtreg&pr=50&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jtreg&pr=50&range=00-01

  Stats: 44 lines in 3 files changed: 35 ins; 5 del; 4 mod
  Patch: https://git.openjdk.java.net/jtreg/pull/50.diff
  Fetch: git fetch https://git.openjdk.java.net/jtreg pull/50/head:pull/50

PR: https://git.openjdk.java.net/jtreg/pull/50