See gh-1885

Issue: SPR-17040

This commit migrates the rest of Spring's test suite to JUnit Jupiter,
except spring-test which will be migrated in a separate commit.

See gh-23451

Migrate all existing JUnit 4 `assert...` based assertions to AssertJ
and add a checkstyle rule to ensure they don't return.

See gh-23022

Migrate all existing `assertThat(..., Matcher)` assertions to AssertJ
and add checkstyle rules to ensure they don't return.

See gh-23022

Organize test imports to expand all '.*' static imports into
fully qualified imports.

This update will allow us to use additional checkstyle rules in
the future, and will also help if we migrate fully to AssertJ.

org.junit.Assert.assertThat() is deprecated as of JUnit 4.13.

See gh-22894

This commit updates URLs to prefer the https protocol. Redirects are not followed to avoid accidentally expanding intentionally shortened URLs (i.e. if using a URL shortener).

# Fixed URLs

## Fixed Success
These URLs were switched to an https URL with a 2xx status. While the status was successful, your review is still recommended.

* [ ] http://www.apache.org/licenses/ with 1 occurrences migrated to:
  https://www.apache.org/licenses/ ([https](https://www.apache.org/licenses/) result 200).
* [ ] http://www.apache.org/licenses/LICENSE-2.0 with 6515 occurrences migrated to:
  https://www.apache.org/licenses/LICENSE-2.0 ([https](https://www.apache.org/licenses/LICENSE-2.0) result 200).

This commit updates URLs to prefer the https protocol. Redirects are not followed to avoid accidentally expanding intentionally shortened URLs (i.e. if using a URL shortener).

# Fixed URLs

## Fixed Success
These URLs were switched to an https URL with a 2xx status. While the status was successful, your review is still recommended.

* [ ] http://www.apache.org/licenses/ with 1 occurrences migrated to:
  https://www.apache.org/licenses/ ([https](https://www.apache.org/licenses/) result 200).
* [ ] http://www.apache.org/licenses/LICENSE-2.0 with 6625 occurrences migrated to:
  https://www.apache.org/licenses/LICENSE-2.0 ([https](https://www.apache.org/licenses/LICENSE-2.0) result 200).

xmlunit 2.1.0 is the latest release for xmlunit.
Most of the xmlunit functionality used within spring-framework
was done through the xmlunit 1.x helper class
`org.custommonkey.xmlunit.XMLAssert`.

As of xmlunit 2.0.0 most of the XML comparison methods are done
through hamcrest matchers exposed by the xmlunit-matchers
library. In some cases during the migration, the matchers
had to be customized with custom `NodeMatcher` or
`DifferenceEvaluator` instances in order to keep the assertions
correct (they were performed with xmlunit 1.x previously).

Issue: SPR-14043

Issue: SPR-14492

Issue: SPR-13188

Issue: SPR-11893

Clean up compiler warnings in the tests of spring-web. This commit
adds type parameters to all the types (mostly `List` and `Map`).

After this commit the only warnings in spring-web left are in
* `MockExpressionEvaluator` this can't be fixed until JSP-EL is fixed
* `Jackson2ObjectMapperFactoryBeanTests#testSetModules` that code
   would never compile with generics.

This renaming more intuitively expresses the relationship between
subprojects and the JAR artifacts they produce.

Tracking history across these renames is possible, but it requires
use of the --follow flag to `git log`, for example

    $ git log spring-aop/src/main/java/org/springframework/aop/Advisor.java

will show history up until the renaming event, where

    $ git log --follow spring-aop/src/main/java/org/springframework/aop/Advisor.java

will show history for all changes to the file, before and after the
renaming.

See http://chrisbeams.com/git-diff-across-renamed-directories