Fixescoder/internal#1035

Or, at least, closes a remaining race that seems pretty likely.

The tests in question write a file, close the file, then execute the file. Sometimes Linux errors saying "text file busy" which means the file is still open for writing.

What I think is going on is:

1. Test_sshConfigProxyCommandEscape goroutine opens the file and begins writing.
2. Some other, unrelated test execs a command, which causes afork() syscall. The child process now has a copy of the file descriptor to our open file.
3. Test_sshConfigProxyCommandEscape goroutine executes the file and gets "text file busy".
4. The child process calls theexec syscall, which closes the file (due toCLOEXEC being set).

The race is very tight because 3 has to happen before 4 (and, 3 involves it's own fork/exec), but it's not impossible on a busy system.

c.f.#14233 which was an earlier attempt to fix this. It only prevented the subtests from running in parallel. When the subtests were all running in parallel, the flake was fairly likely because you've got all this fork() activity happening at the same time. But, since the main test was in parallel there is still a chance a totally different test isfork'ing at in inopportune time.