test: Use associative arrays to track external prereqs

Previously, the test framework generated a variable name for each
external prereq as a poor man's associative array.  Unfortunately,
prereqs names may not be legal variable names, leading to
unintelligible bash errors like
  test_missing_external_prereq_emacsclient.emacs24_=t: command not found

Using proper associative arrays to track prereqs, in addition to being
much cleaner than generating variable names and using grep to
carefully construct unique string lists, removes restrictions on
prereq names.

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