fix(language-service): reset MockHost after every spec instead of creating new LS (#33200)

This commit speeds up the tests by calling `MockHost.reset()` in
`beforeEach()` instead of destroying the entire language service and
creating a new one. The creation of a new language service instance is
expensive due to the need to initialize many core Symbols when creating
a new program.

This speeds ups the test (on my local machine) from 35 secs to 15 secs.

PR Close #33200

packages/language-service/test/BUILD.bazel

@@ -13,7 +13,6 @@ ts_library(
         "//packages/compiler",
         "//packages/compiler-cli/test:test_utils",
         "//packages/language-service",
-        "@npm//reflect-metadata",
         "@npm//typescript",
     ],
 )