test/subprocess: add missing space in SubshellKilled subcommand

9abf2388f24 rewrote "sh -c cat" as SHELL "-c cat", which concatenates
to "/bin/sh-c cat", so the subshell tries to execute a non-existent
"/bin/sh-c". The test still passed because the SIGTERM sent by kill()
usually wins the race against the shell startup; under CI load the
shell occasionally prints

  /bin/sh: line 1: /bin/sh-c: No such file or directory

into the stderr pipe first, ASSERT_TRUE(buf.empty()) fails, and the
skipped join() turns this into a SIGABRT in ~SubProcess().

Signed-off-by: Sun Yuechi <sunyuechi@iscas.ac.cn>