Add a TestResourcesScope annotation

[melix]

By default, test resources are shared between all tests. This means, in particular, that a single container will be spawned if multiple tests require the same property. For example, we would have a single MySQL container for the whole test suite, which makes testing significantly faster, avoiding containers to be started and shutdown in each test.

However, in some situations it may be required to isolate tests from each other, and a test may want to work with its own, isolated container.

This commit introduces a new TestResourcesScope annotation which can be put added at a test class level, which defines the scope of the test resources used in that test. A scope can be shared by multiple tests, and you can see the general case (all tests share the same resources) as the "root scope". A scope is closed whenever the last test which works in that scope is finished.

Before, doing this required assigning a test property to the test and manually counting the number of tests which are executed, then calling the test resources client directly when all tests are executed, which is both error prone and imprecise (in case only a subset of the tests are executed).

Because of implementation limitations, the way the scope is recognized is by using a thread local, which has a couple consequences:

1. it only works properly if tests are executed in a single JVM.
2. tests which use custom threads and execute the application in that thread wouldn't see the scope and have to rely on using the client

This significantly simplifies the test setup in Micronaut Data. Take this existing code:

class MySqlJoinSpec extends AbstractJoinSpec implements MySQLTestPropertyProvider {
...
}

The MySQLTestPropertyProvider is implemented as:

trait MySQLTestPropertyProvider implements SharedTestResourcesDatabaseTestPropertyProvider {

    @Override
    Dialect dialect() {
        Dialect.MYSQL
    }

    @Override
    int sharedSpecsCount() {
        return 7
    }
}

And SharedTestResourcesDatabaseTestPropertyProvider:

trait SharedTestResourcesDatabaseTestPropertyProvider implements TestResourcesDatabaseTestPropertyProvider {

    abstract int sharedSpecsCount()

    def cleanupSpec() {
        int testsCompleted = DbHolder.DB_TYPE_TEST_COUNT.compute(dbType(), (dbType, val) -> val ? val + 1 : 1)
        if (testsCompleted == sharedSpecsCount()) {
            try {
                TestResourcesClient testResourcesClient = TestResourcesClientFactory.fromSystemProperties().get()
                testResourcesClient.closeScope(dbType())
            } catch (Exception e) {
                // Ignore
            }
        }
    }
}

And finally TestResourcesDatabaseTestPropertyProvider:

trait TestResourcesDatabaseTestPropertyProvider implements TestPropertyProvider {

   ...

    Map<String, String> getDataSourceProperties(String dataSourceName) {
        def prefix = 'datasources.' + dataSourceName
        return [
                'micronaut.test.resources.scope': dbType(),
                (prefix + '.db-type')           : dbType(),
                (prefix + '.schema-generate')   : schemaGenerate(),
                (prefix + '.dialect')           : dialect(),
                (prefix + '.packages')          : packages(),
        ] as Map<String, String>
    }

}

With this PR, we can simplify the code a lot, by getting rid of SharedTestResourcesDatabaseTestPropertyProvider altogether, and annotating TestResourcesDatabaseTestPropertyProvider:

@TestResourcesScope(namingStrategy = ScopeNamingStrategy.PackageName)
trait TestResourcesDatabaseTestPropertyProvider implements TestPropertyProvider {
    ...
    Map<String, String> getDataSourceProperties(String dataSourceName) {
        def prefix = 'datasources.' + dataSourceName
        return [
                (prefix + '.db-type')           : dbType(),
                (prefix + '.schema-generate')   : schemaGenerate(),
                (prefix + '.dialect')           : dialect(),
                (prefix + '.packages')          : packages(),
        ] as Map<String, String>
    }

}

It is no longer required to explicitly call the client, nor to set the scope in the test property provider, and the scope can be automatically derived from the package of the test class.

@graemerocher if this is approved I will create a new PR documenting this feature and the one it depends on (#231)

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