Data races in TestBlockJob fields in test-block-iothread
Description of problem:
A data race in the access of `TestBlockJob` fields in `tests/unit/test-block-iothread.c` was identified using TSAN.
Steps to reproduce:
```sh
QEMU_BUILD_DIR=<path to the QEMU build directory>
QEMU_DIR=<path to the QEMU repository directory>
configure --enable-tsan --cc=clang --cxx=clang++ --enable-trace-backends=ust --enable-fdt=system --disable-slirp
make tests/unit/test-block-iothread
MALLOC_PERTURB_=67 G_TEST_SRCDIR=$QEMU_BUILD_DIR/tests/unit G_TEST_BUILDDIR=$QEMU_BUILD_DIR/tests/unit $QEMU_BUILD_DIR/tests/unit/test-block-iothread --tap -k
```