Today there is no way to mark a test as “run only on request” without
every user remembering -LE or -E. DISABLED never runs, even with -R.
Proposal: test property MANUAL. A MANUAL test is skipped unless a -R
regex is given and selects it. -E/-L/-LE/-I/-U do not select it on
their own.
Open questions: the name (MANUAL vs DISABLED_BY_DEFAULT …), whether
–tests-from-file should count as explicit, how DEPENDS on a manual
test should behave, and fixtures (current patch: manual setup/cleanup
tests still run when a selected test requires the fixture).