RFR: 8355443: [java.io] Use @requires tag instead of exiting based on File.separatorChar value

[Brian Burkhalter]

For tests of the `java.io` package, instead of doing this

    public static void main(String[] args) throws Exception {
        if (File.separatorChar != '\') {
            /* This test is only valid on win32 systems */
            return;
        }

do this

   @requires (os.family == "windows")

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

Commit messages:
 - 8355443: [java.io] Use @requires tag instead of exiting based on File.separatorChar value

Changes: https://git.openjdk.org/jdk/pull/24838/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24838&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8355443
  Stats: 62 lines in 10 files changed: 10 ins; 35 del; 17 mod
  Patch: https://git.openjdk.org/jdk/pull/24838.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24838/head:pull/24838

PR: https://git.openjdk.org/jdk/pull/24838