Do not let a shrugging sysconf turn a workstation into a tiny machine

Three from the review of the change before this one.

sysconf answers -1 for a limit it holds to be indeterminate, and CPython
hands that back rather than raising, so the page count times the page size
came out negative. A negative is truthy, so it went past the check for a
machine nothing could be read from and floored at half a gigabyte: a 64 GB
workstation was told every model past 512 MB was too big for it, the
suggestion dropped to small-q5_1, and the machine line read "Memory:
-4096 B". Anything not positive is now the unknown machine it always was.

The memory is read once and kept. It does not change while Dikte runs, and
a thirty row list asked seventy times per draw, which on the Mac path is
seventy processes started on the interface thread every time a download
finished, a model was deleted or a publisher changed.

And "test-" is matched as a plain substring, so it was also inside
"Latest-" and dropped a publisher nothing is wrong with. Anchored the way
every other mark in that list already is.

tests/support.py

@@ -24,6 +24,7 @@ from unittest import mock
 
 from dikte import assistant
 from dikte import config as cfg
+from dikte import ggml
 from dikte import i18n
 from dikte import update
 
@@ -95,6 +96,10 @@ class DikteTest(unittest.TestCase):
         i18n.set_language("en")
         self.addCleanup(i18n.set_language, "en")
 
+        # Read once and kept for the life of the process, which across a test
+        # run means one test's machine answering for the next one's.
+        self.patch_attr(ggml, "_MEMORY", None)
+
         # cli.launch_gui replaces this process with the application when no
         # instance is running. A test that reaches it would take the whole run
         # with it and hang, so it fails loudly here instead.