RFR: 7903097: jtreg could implement ToolProvider [v4]

[Christian Stein]

> Implement `ToolProvider`, now that the main `jtreg` tool requires JDK >= 9.

Christian Stein has updated the pull request incrementally with one additional commit since the last revision:

  Apply suggested changes

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Changes:
  - all: https://git.openjdk.java.net/jtreg/pull/54/files
  - new: https://git.openjdk.java.net/jtreg/pull/54/files/8e50650a..8c8bf02e

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jtreg&pr=54&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jtreg&pr=54&range=02-03

  Stats: 89 lines in 4 files changed: 43 ins; 40 del; 6 mod
  Patch: https://git.openjdk.java.net/jtreg/pull/54.diff
  Fetch: git fetch https://git.openjdk.java.net/jtreg pull/54/head:pull/54

PR: https://git.openjdk.java.net/jtreg/pull/54