ctest: rename TRACK to GROUP

Update command-line options, script variables, and documentation to use
the term "group" instead of "track". The old terms are still available
for now, but they are now undocumented.

This makes our terminology more consistent with CDash. The goal of this
change is to make it more clear to our users how CTest and CDash interact
with each other.
50 jobs for group_not_track
Status Name Job ID Coverage
  External
passed cdash

passed cmake-dash3win10-windows-release-ninja

passed cmake-hythloth-linux-debug-makefiles+sphinx

passed cmake-minoc-linux-release-lint-iwyu-tidy

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passed cmake-terfin-windows-release-vs2019

passed ghostflow-check-master

passed ghostflow-stager

passed cmake-dash3win10-windows-release-ninja

passed cmake-dash3win10-windows-release-ninja

00:19:55

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passed cmake-hythloth-linux-debug-makefiles+sphinx

passed cmake-hythloth-linux-debug-makefiles+sphinx

00:46:21

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passed cmake-minoc-linux-release-lint-iwyu-tidy

00:19:55

passed cmake-minoc-linux-release-lint-iwyu-tidy

passed cmake-moonshade-osx-debug-ninja

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passed cmake-moonshade-osx-debug-ninja

00:13:35

passed cmake-moonshade-osx-release-makefiles

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00:14:20

passed cmake-moonshade-osx-release-makefiles

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passed cmake-terfin-windows-release-vs2019

passed cmake-terfin-windows-release-vs2019

00:39:15

passed cmake-terfin-windows-release-vs2019

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passed ghostflow-stager

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