selftests: coredump: Raise timeout to 2 minutes

selftests: coredump: Raise timeout to 2 minutes

The test's runtime (nearly 20s) is dangerously close to the limit (30s) on
qemu-system-riscv64:

$ time ./stackdump_test > /dev/null
real	0m19.210s
user	0m0.077s
sys	0m0.359s

There could be machines slower than qemu-system-riscv64. Therefore raise
the test timeout to 2 minutes to be safe.

Fixes: 15858da53542 ("selftests: coredump: Add stackdump test")
Signed-off-by: Nam Cao <namcao@linutronix.de>
Link: https://lore.kernel.org/dd636084d55e7828782728d087fa2298dcab1c8b.1744383419.git.namcao@linutronix.de
Signed-off-by: Christian Brauner <brauner@kernel.org>