RFR: 8264539: Improve failure message of java/nio/file/WatchService/SensitivityModifier.java

[Brian Burkhalter]

Please consider this request to improve the message printed when no `ENTRY_MODIFY` events are received. In the polling implementation of `WatchService` currently used on macOS, it would be useful to know the sensitivity of polling in effect for the failing case. It might indicate, for example, that low sensitivity should not be used in the test.

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Commit messages:
 - 8264539: Improve failure message of java/nio/file/WatchService/SensitivityModifier.java

Changes: https://git.openjdk.java.net/jdk/pull/3290/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3290&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264539
  Stats: 22 lines in 1 file changed: 15 ins; 0 del; 7 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3290.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3290/head:pull/3290

PR: https://git.openjdk.java.net/jdk/pull/3290