language server tests do not unnecessarily re-assemble fat jar

[Akirathan]

Follow-up of #8467.

Pull Request Description

language-server / test used to depend on updateLibraryManifest, which, in turn, depends on runtime-fat-jar / assembly, which is an expensive operation, that is not cache, so it is invoked even if no sources were changed. In this PR, I am trying to remove this dependency and thus speed up the language-server test invocation.

Important Notes

Previous behavior (without changing any sources):

sbt:enso> language-server/testOnly *SuggestionsHandlerSpec
[warn] Caching disabled because of a custom merge strategy: 'Discard all module-info except for module-info from project runtime'
[info] 5453 file(s) merged using strategy 'First' (Run the task at debug level to see the details)
[info] 12 file(s) merged using strategy 'Concat' (Run the task at debug level to see the details)
[info] 5 file(s) merged using strategy 'Discard all module-info except for module-info from project runtime (Custom)' (Run the task at debug level to see the details)
[info] 48 file(s) merged using strategy 'Discard' (Run the task at debug level to see the details)
[info] Built: runtime.jar
[info] Jar hash: 9d75ba53f9fc7a8419bc32bddce1263bb9128521
[info] Assembly jar up to date: runner.jar
SLF4J: Attempting to load provider "org.enso.logger.TestLogProvider" specified via "slf4j.provider" system property
[info] SuggestionsHandlerSpec:

Checklist

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

• The documentation has been updated, if necessary.
• Screenshots/screencasts have been attached, if there are any visual changes. For interactive or animated visual changes, a screencast is preferred.
• All code follows the
  Scala,
  Java,
  and
  Rust
  style guides. In case you are using a language not listed above, follow the Rust style guide.
• All code has been tested:
  • Unit tests have been written where possible.
  • If GUI codebase was changed, the GUI was tested when built using ./run ide build.