`spack install` terminal output handling in foreground/background (#15723)

Makes the following changes:

* (Fixes #15620) tty configuration was failing when stdout was 
  redirected. The implementation now creates a pseudo terminal for
  stdin and checks stdout properly, so redirections of stdin/out/err
  should be handled now.
* Handles terminal configuration when the Spack process moves between
  the foreground and background (possibly multiple times) during a
  build.
* Spack adjusts terminal settings to allow users to to enable/disable
  build process output to the terminal using a "v" toggle, abnormal
  exit cases (like CTRL-C) could leave the terminal in an unusable
  state. This is addressed here with a special-case handler which
  restores terminal settings.

Significantly extend testing of process output logger:

* New PseudoShell object for setting up a master and child process
  and configuring file descriptor inheritance between the two
* Tests for "v" verbosity toggle making use of the added PseudoShell
  object
* Added `uniq` function which takes a list of elements and replaces
  any consecutive sequence of duplicate elements with a single
  instance (e.g. "112211" -> "121")

Co-authored-by: Adam J. Stewart <ajstewart426@gmail.com>

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