Previously, each test build script used a property to configure its
classpath. This commit simplifies the tests by setting the classpath
once on the GradleRunner, thereby removing the need for it to be set in
each test script. It also allows, and, in fact, requires, the use of
the plugins block to apply the Boot plugin under test.

Unfortunately, this doesn't work for the tests for the reaction to the
Kotlin plugin. See the comments in the GradleBuild class and in each
KotlingPluginActionIntegrationTests build script.

See gh-14680