Adapt eglot tests on emba

* test/infra/Dockerfile.emba (emacs-eglot): Install rust-src.

* test/infra/gitlab-ci.yml (test-eglot): Extend EMACS_EXTRAOPT.

* test/lisp/progmodes/eglot-tests.el
(eglot-test-rust-completion-exit-function): Don't skip on emba.

test/infra/Dockerfile.emba

@@ -71,7 +71,7 @@ RUN apt-get update && \
 # Install clangd, tsserver, rust-analyzer.
 RUN apt-get update && \
     apt-get install -y --no-install-recommends -o=Dpkg::Use-Pty=0 \
-      clangd npm node-typescript pipx rust-analyzer cargo \
+      clangd npm node-typescript pipx rust-analyzer cargo rust-src \
     && rm -rf /var/lib/apt/lists/*
 
 RUN npm install -g typescript-language-server

test/infra/gitlab-ci.yml

@@ -327,7 +327,7 @@ test-eglot:
       check-expensive
       TEST_HOME=/root
       LOGFILES="lisp/progmodes/eglot-tests.log"
-      EMACS_EXTRAOPT='--eval \(package-install\ \(quote\ company\)\) --eval \(package-install\ \(quote\ yasnippet\)\) --eval \(setopt\ package-selected-packages\ \(quote\ \(company\ yasnippet\)\)\) --eval \(setq\ exec-path\ \(cons\ \"/root/.local/bin\"\ exec-path\)\)'
+      EMACS_EXTRAOPT='--eval \(package-install\ \(quote\ company\)\) --eval \(package-install\ \(quote\ yasnippet\)\) --eval \(setopt\ package-selected-packages\ \(quote\ \(company\ yasnippet\)\)\) --eval \(setq\ exec-path\ \(cons\ \"/root/.local/bin\"\ exec-path\)\) --eval \(setenv\ \"PATH\"\ \(concat\ \(getenv\ \"PATH\"\)\ \":/root/.local/bin\"\)\)'
 
 build-image-tree-sitter:
   stage: platform-images

test/lisp/progmodes/eglot-tests.el

@@ -776,7 +776,6 @@ directory hierarchy."
   ;; This originally appeared in github#1339
   (skip-unless (executable-find "rust-analyzer"))
   (skip-unless (executable-find "cargo"))
-  (skip-unless (not (getenv "EMACS_EMBA_CI")))
   (eglot--with-fixture
       '(("cmpl-project" .
          (("main.rs" .