RFR: 8264140: Add -Djava.security.manager=allow to JavaFX tests calling setSecurityManager

[Kevin Rushforth]

The security manager is being deprecated for removal by [JEP 411](https://openjdk.java.net/jeps/411), which is proposed for JDK 17. The implementation of JEP 411 is out for review here: openjdk/jdk#4073.

Along with the deprecation of most security manager classes and methods, the default value of the `java.security.manager` system property will be treated as "`disallow`". This means that the deprecated `System::setSecurityManager` method will throw `UnsupportedOperationException` unless `-Djava.security.manager=allow` is set on the command line. A few system tests and javafx.web unit tests set a security manager and will fail without setting that system property.

The fix is to set the `-Djava.security.manager=allow` in `build.gradle` when running tests in the `:systemTests` and `:web` projects. Support for setting `java.security.manager` to "`allow`" was added in JDK 12, so we also check that the boot JDK is >= 12 (the check is needed because JDK 11 is the minimum boot JDK).

As part of this PR I am removing the legacy `ServiceWithSecurityManagerTest` to address [JDK-8234175](https://bugs.openjdk.java.net/browse/JDK-8234175). `ServiceWithSecurityManagerTest` is the only unit test in the `javafx.graphics` module that sets a security manager, and the test has skipped via `@Ignore` since FX 8 with the comment that the test "appears to be bogus". Since there are no active `javafx.graphics` tests that set the security manager, I don't see the need to set `-Djava.security.manager=allow` for `javafx.graphics` tests. Better to just remove the test.

I have done a full build / test using JDK 15 as a boot JDK. I then did a full build / test using a locally-built JDK 17-ea with the fix for JEP 411, and setting `JDK_HOME` to that JDK. With that, I can confirm that the tests that set the security manager fail without this fix and pass with this fix.

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Commit messages:
 - 8264140: Add -Djava.security.manager=allow to JavaFX tests calling setSecurityManager
 - 8234175: Remove obsolete ServiceWithSecurityManagerTest unit test

Changes: https://git.openjdk.java.net/jfx/pull/510/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=510&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264140
  Stats: 139 lines in 2 files changed: 11 ins; 128 del; 0 mod
  Patch: https://git.openjdk.java.net/jfx/pull/510.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/510/head:pull/510

PR: https://git.openjdk.java.net/jfx/pull/510