gosh: Fix flaky test

The sub-process handling is a little big fragile and leads to flaky test
results, e.g.

--- FAIL: TestCleanupProcessGroup (1.12s)
shell_test.go:45: got 4, want 5

Rewrote the test to only check whether the child processes are still
running.

Fixes https://github.com/vanadium/build/issues/55

Change-Id: I2a11bdec408d33ed86bb0fdd6435c7dd231f5f3b
1 file changed