There seems to be a conflict between StringOverrides and SimpleTest, especially when strings used in forms (like 'Log in') are overridden. Issue is described in #1139298: Static $custom_strings in t() breaks forms in tests. The issue is probably mostly on the SimpleTest side, but I want to bring it to your attention and record it in the queue in case others run into it.
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Comment #1
thebuckst0p commentedUntil that bug is fixed, and it might be core to D6, the workaround is to disable StringOverrides and set $conf in code.