Fix tclapp::install junit failure in appinit sandbox pass

[mumallaeng]

Fixes #823

Problem

tclapp::install junit fails with:

unknown namespace in import pattern "::msgcat::mcload"

Root cause

appinit::load_app probes an app for its exported procs by sourcing its files in a throwaway interpreter where package require is stubbed to a no-op. JUnitAssertionMgr.tcl's init proc runs at module load time and calls clock format, which lazily sources Tcl's clock.tcl and needs msgcat. Since package require is stubbed in that interpreter, msgcat never actually loads there, so namespace import ::msgcat::mcload fails.

That probe interpreter is discarded right after collecting the exported proc names, so nothing computed in init during that pass is actually used.

Fix

Guard the clock format call with catch, falling back to an empty timestamp on failure. Harmless during the probe pass (result is discarded); the real load path (via junit.tcl's package mechanism, unstubbed) is unaffected.

Testing

• Reproduced the exact reported error on Vivado 2026.1 (tclapp::install junit)
• Confirmed the fix resolves it on the same Vivado 2026.1 install
• Root cause isolated and verified in a minimal standalone Tcl 8.6 reproduction of appinit's sandbox pattern