JDK 12 RFR of JDK-8219254: Update explicit uses of latest source/target in langtools tests to a property

[Jonathan Gibbons]

On 02/19/2019 10:23 AM, Joe Darcy wrote:
>
> Besides the future work files under diags/examples, after the changes 
> in the webrev, the remaining uses of "-source 13" are in
>
>     ./tools/javac/expswitch/ExpSwitchNestingTest.java

Because this is a singleton file, it might be easiest to convert it to a 
"normal" jtreg test -- still using TestNG of course -- by removing the 
TEST.properties file, and using the standard test description tags, 
ending with `@run testng ...` to run the test.

-- Jon