test(phpunit): fix PHPUnit different CWD with a custom command

As per https://github.com/recca0120/vscode-phpunit/issues/385#issuecomment-3947799942

Also needed to set `XDEBUG_MODE`.

Generally works, but now I get test failures.

.vscode/settings.json

@@ -2,14 +2,17 @@
     "files.associations": {
         "**/cfg/conf*.php": "ini"
     },
+    "phpunit.environment": {
+        "XDEBUG_MODE": "coverage"
+    },
     "phpunit.phpunit": "${userHome}/.composer/vendor/bin/phpunit",
+    "phpunit.command": "/bin/sh -c 'cd ${workspaceFolder}/tst && ${php} ${phpunit} ${phpunitargs}'",
     "phpunit.args": [
         "--configuration",
         "${workspaceFolder}/tst/phpunit.xml",
         "--bootstrap",
         "${workspaceFolder}/tst/Bootstrap.php"
     ],
-    "mochaExplorer.mochaPath": "js/node_modules/mocha",
     "mochaExplorer.pkgFile": "js/package.json",
     "mochaExplorer.files": "js/test/*.js"
 }