make(1): fix bug in evaluation of indirect variable modifiers

Since 2020-09-22, when indirect variable modifiers were applied to a
variable expression based on an undefined variable, these modifiers did
not change the state of a variable expression from undefined to defined.
The modifiers that do this are :D, :U, :L, :P. Minimal example:

${VARNAME:${:UL}}

The :L modifier from the inner expression sets the value of the outer
expression to its variable name, in this case "VARNAME". The outer
expression was not marked as being defined though, which resulted in a
"Malformed conditional" error.

In the commit from 2020-09-22, vardebug.exp had changed a lot, and I had
not inspected the change closely. The important detail was in lines 56
and 60, where VAR_JUNK|VAR_KEEP changed into VEF_UNDEF, thereby losing
the VEF_DEF bit.

make(1): demonstrate bug in evaluation of indirect variable modifiers

Reported by sjg.

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.