tests/make: extend test suite, move old tests to 2020 scheme

The tests from envfirst.mk are now in opt-env.mk.
The tests from modword.mk are now in varmod-select-words.mk.

make(1): add dummies for fine-grained tests, one per single feature

The test names have been derived from the current manual page.

All these tests are dummies right now, and the code from the existing
tests will be moved into the new tests step by step.

This is done to prevent modmisc, escape, varmod-edge and varmisc from
growing without any bounds, and to reduce the side-effects of one test
to the others.