jshell tool: /edit with external editor leaks files in /tmp

Was: langtools/test/jdk/jshell/ToolRetainTest.java leaks files in /tmp

langtools/test/jdk/jshell/ToolRetainTest.java leaks files in /tmp.

If the files are being written by jshell, they should be cleaned up.

If the files are being written by the test itself, it would be more appropriate to use the jtreg scratch directory (i.e. current directory, when test is running.)

See attached script for running tests one at a time and checking for jshell files in /tmp.