RFR: 8282042: [testbug] FileEncodingTest.java depends on default encoding

[Tyler Steele]

FileEncodingTest expects all non-Windows platforms will have `Charset.defaultCharset().name()` set to US-ASCII when file.encoding is set to COMPAT. This assumption does not hold for AIX where it is ISO-8859-1.

According to [JEP-400](https://openjdk.java.net/jeps/400), we should expect  `Charset.defaultCharset().name()` to equal `System.getProperty("native.encoding")` whenever the COMPAT flag is set. From JEP-400: "... if file.encoding is set to COMPAT on the command line, then the run-time value of file.encoding will be the same as the run-time value of native.encoding...". So one way to resolve this is to choose the value for each system from the native.encoding property.

With these changes however, my test systems continue to fail. 

- AIX reports: Default Charset: ISO-8859-1, expected: ISO8859-1
- Linux/Z reports: Default Charset: US-ASCII, expected: ANSI_X3.4-1968
- Linux/PowerLE reports: Default Charset: US-ASCII, expected: ANSI_X3.4-1968

Note that the expected value is populated from native.encoding.

This implies more work to be done. It looks to me that some modification to java_props_md.c may be needed to ensure that the System properties for native.encoding return [canonical names](http://www.iana.org/assignments/character-sets). 

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A tempting alternative is to set the expected value for AIX to "ISO-8859-1" in the test explicitly, as was done for the Windows expected encoding prior to this proposed change. The main advantage to this alternative is that it is quick and easy, but the disadvantages are that it fails to test that COMPAT behaves as specified in JEP-400, and the approach does not scale well if it happens that other systems require other cases. I wonder if this is the reason non-English locals are excluded by the test.

Proceeding with this change and the work implied by the new failures it highlights goes beyond the scope of what I thought was a simple testbug. So I'm opening this up for some comments before proceeding down the rabbit hole of further changes. If there is generally positive support for this direction I'm happy to make the modifications necessary to populate native.encoding with canonical names. As I am new to OpenJDK, I am especially looking to ensure that changing the value returned by native.encoding will not have unintended consequences elsewhere in the project.

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Commit messages:
 - Changes FileEncodingTest test to delegate behaviour of -Dfile.encoding=COMPAT to

Changes: https://git.openjdk.java.net/jdk/pull/7525/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7525&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8282042
  Stats: 8 lines in 1 file changed: 0 ins; 4 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7525.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7525/head:pull/7525

PR: https://git.openjdk.java.net/jdk/pull/7525