composer update したところ、ext-dom がないから PHPUnitが入れられないようです。

$ composer update
Loading composer repositories with package information
Updating dependencies (including require-dev)
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - phpunit/phpunit 8.5.x-dev requires ext-dom * -> the requested PHP extension dom is missing from your system.
...

php-xml をインストール

# aptitude update
# aptitude install php-xml

再度 composer update する

今度は、インストールが進み、vendor もインストールされた。

$ composer update
Loading composer repositories with package information
Updating dependencies (including require-dev)
Package operations: 87 installs, 0 updates, 0 removals
As there is no 'unzip' command installed zip files are being unpacked using the PHP zip extension.
This may cause invalid reports of corrupted archives. Besides, any UNIX permissions (e.g. executable) defined in the archives will be lost.
Installing 'unzip' may remediate them.
  - Installing symfony/polyfill-ctype (v1.14.0): Loading from cache
...