Remove use of Thread.stop from jshell tests

The jshell tests have an infrastructure class named UITesting that uses Thread.stop. This needs to be updated to remove the dependency on Thread.stop, otherwise there are test failures like this:

java.lang.UnsupportedOperationException
at java.base/java.lang.Thread.stop(Thread.java:1658)
at UITesting.doRunTest(UITesting.java:127)
at HistoryUITest.testPrevNextSnippet(HistoryUITest.java:51)

Right now, there seem to be 7 jshell tests that make use of this:

jdk/jshell/ToolTabSnippetTest.java
jdk/jshell/ToolTabCommandTest.java
jdk/jshell/ToolShiftTabTest.java
jdk/jshell/ToolMultilineSnippetHistoryTest.java
jdk/jshell/PasteAndMeasurementsUITest.java
jdk/jshell/IndentUITest.java
jdk/jshell/HistoryUITest.java