tests: fix comment declaring runtime in rv64 interruptedmemory test

The test attempts to trigger a regression for arount 30s. However, a
comment just before the computation of the target wall clock time falsly
declares the run time to be around 60s.

This was the case already when we introduced the test in

    5241645c47a9987f4fcc65bab303a444966b7942
    (tests: add test with interrupted memory accesses on rv64)

Signed-off-by: Julian Ganz <neither@nut.email>
Reviewed-by: Daniel Henrique Barboza <daniel.barboza@oss.qualcomm.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>