Start child processes one at a time; run gcov without a directory

Reading the coverage profiles on several threads broke an assumption of
ChildProcess::Start on POSIX: it changes the working directory of the
whole process around posix_spawn. Two gcov runs started at once could
both run in one test's directory, so on the GCC job the map of
"Subtracts one" showed what "Adds one" reached.

- ChildProcess::Start holds a lock while a child starts, on POSIX and
  Windows, so the directory change and the handles a child inherits never
  overlap with another start.
- gcov gets the absolute path of the data file and no working directory,
  so no directory changes while profiles are read in parallel.

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