preamble_size: Fix variability of command & preamble(?) size

Previously the command size could very depanding on the current username, hostname, and process pid.

Before
```
SSH command size: 759
Preamble (mitogen.core + econtext) size: 18227 (17.80KiB)
...
```

After
SSH command size: 755
Preamble (mitogen.core + econtext) size: 18227 (17.80KiB)
...
```

Author: moreati

docs/changelog.rst

@@ -23,6 +23,7 @@ In progress (unreleased)
 
 * :gh:issue:`1306` CI: Report sudo version on Ansible targets
 * :gh:issue:`1306` CI: Move sudo test users defaults into ``/etc/sudoers.d``
+* :gh:issue:`1306` preamble_size: Fix variability of measured command size
 
 
 v0.3.27 (2025-08-20)

preamble_size.py

@@ -31,7 +31,11 @@ def header(self):
 
 router = mitogen.master.Router()
 context = mitogen.parent.Context(router, 0)
-options = mitogen.ssh.Options(max_message_size=0, hostname='foo')
+options = mitogen.ssh.Options(
+    hostname='foo',
+    max_message_size=0,
+    remote_name='alice@host:1234',
+)
 conn = mitogen.ssh.Connection(options, router)
 conn.context = context