Fix dependencies on library-manager

[hubertp]

Pull Request Description

This change fixes a dependency on library-manager that was causing ClassNotFoundException when running tests:

[info] LibraryDownloadTest:
[info] DownloadingLibraryCache
SLF4J(I): Attempting to load provider "org.enso.logging.service.logback.test.provider.TestLogProvider" specified via "slf4j.provider" system propertyTests 0s
SLF4J(E): Failed to instantiate the specified SLF4JServiceProvider (org.enso.logging.service.logback.test.provider.TestLogProvider)
SLF4J(E): Reported exception:
java.lang.ClassNotFoundException: org.enso.logging.service.logback.test.provider.TestLogProvider
	at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:641)
	at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:188)
	at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:526)
	at org.slf4j.LoggerFactory.loadExplicitlySpecified(LoggerFactory.java:226)
	at org.slf4j.LoggerFactory.findServiceProviders(LoggerFactory.java:122)

While I was there I noticed that library-manager-test is rather obsolete if only we set the dependencies between projects right.
Hence, classes of library-manage-test are moved to library-manager's test directory and the former is dropped completely.

Important Notes

Other -test projects should be set up in a similar fashion.

Checklist

Please ensure that the following checklist has been satisfied before submitting the PR:

• All code follows the
  Scala,
  Java,
  TypeScript,
  and
  Rust
  style guides. In case you are using a language not listed above, follow the Rust style guide.

[Akirathan]

I assume this fixes the local sbt library-manager/test command. I also assume that on the CI, everything is fine, since we CI does sbt buildEngineDistribution before it executes any test.