RISC-V: process sandbox output for timestamps

What

Automatically determine TPS from sandbox logs.

'Balance checking' to follow in a subsequent MR

Why

To avoid adding it up manually each time

How

Ensure the expected number of executions are present in level 2, and use the timings to determine the TPS.

Manually testing the MR

$ cd src/riscv/jstz
$ make build
$ ./inbox-bench generate --transfers 4
$ cd ..
$ make riscv-sandbox
$ LOG=$(mktemp)
$ ./riscv-sandbox rvemu \
    --input ../../tezt/tests/riscv-tests/hermit-loader \
    --initrd jstz/target/riscv64gc-unknown-hermit/release/jstz \
    --inbox-file ./jstz/inbox.json \
    --timings > $LOG
$ cd jstz
$ ./inbox-bench results --inbox-file inbox.json --log-file $LOG
4 FA2 transfers took 19.118029432s @ 0.209 TPS