test(sharness): improve CAR tests to remove some potential races

[Stebalien]

• Instead of ensuring that we GC nothing, verify that the repo validates.
• Avoid a race between removing the fifos and trying to read from them.
• Avoid all 'bash -c' invocations. Instead, make liberal use of subshells and wait.

Hopefully fixes #7123.

[ribasushi]

Let's merge this as is
I think I found the lock - it is in the part that got removed
I'm, putting together another PR, but it might take (quite) a while

[ribasushi]

This is what the test was supposed to look like, and that doesn't currently work even though it should. Looking...

        # part 2 goes first and blocks waiting on ipfs
        cat ../t0054-dag-car-import-export-data/lotus_devnet_genesis_shuffled_nulroot.car > pipe_devnet &

        # ipfs import starts and blocks on part 1
        ipfsi 0 dag import \
          pipe_testnet \
          pipe_devnet \
          ../t0054-dag-car-import-export-data/combined_naked_roots_genesis_and_128.car \
        2>&1 &

        # part 1 unblocks everything
        cat ../t0054-dag-car-import-export-data/lotus_testnet_export_128_shuffled_nulroot.car > pipe_testnet &

        # remove pipes right away
        rm -f pipe_devnet pipe_testnet