tests/make: force expected lines to be listed in execution order

The diagnostics from the program to check the "expect" lines in the unit
test files were hard to follow since the "out-of-order" lines were
confusing. For out-of-order lines, state where they should be placed
instead.

make: reduce line length in error messages

The error messages for deeply nested parse errors were hard to decipher,
due to the large amount of text. Split these messages into individual
lines, just as in the backtrace for .include files and .for loops. This
unified backtrace makes the output more uniform.

make: add more context information to error messages

In case of a parse error or evaluation error, print the variable value
in addition to the variable name, to see the effects of previous
expression modifiers.

In nested make calls, print the current directory at the bottom of a
stack trace, as that information is otherwise hard to get in a parallel
build spanning multiple directories.

make: error out on syntax error in conditions in ':?then:else' modifier

The 'Error' function only reports errors but does not affect the exit
status, the 'Parse_Error' function does, while providing more details to
find the cause of the syntax error.

branches: 1.6.2;
make: remove redundant text from diagnostic about bad '?:' modifier

tests/make: replace 'variable expression' with 'expression'

Each expression is based on a variable, there's no need for the
verbosity. The wording in make's diagnostics will be changed in a
follow-up commit.

tests/make: clean up comments in tests

make(1): clean up some tests

Fix some mistakes in comments.

Improve some comments to be easier understandable.

Inline variables that are irrelevant for the test at hand (RUN,
MAKE_CMD).

Remove references to functions and constants that have since been removed
or renamed.

make(1): add test for undefined (or empty) variable in :? modifier

usr.bin/make: add test case for lazy conditions