Some tests fail when run with JCov

When running tests with JCov, a number of tests fail because some env variables used to run tests under jcov cause the runtime to emit non-suppressible warnings. These warnings can affect tests that check the output to be as expected.

Many (but not all) of the tests can be fixed by using the latest version of jtreg. But some test failures remain. The list is currently:
jdk/jshell/JavadocTest.java
tools/javac/EarlyAssert.java
tools/javac/T8132562/ClassPathWithDoubleQuotesTest.java
tools/javac/launcher/SourceLauncherTest.java
tools/javac/modules/RequiresStaticTest.java
tools/javac/options/smokeTests/OptionSmokeTest.java
tools/javac/platform/PlatformProviderTest.java
tools/jdeps/modules/CheckModuleTest.java