tests: Add variable expansion tests

There are some corner cases in variable expansion which would be nice to
capture before going and rewriting the variable expansion code.
tests: Test string(CONFIGURE) expansion with escaped quotes

*sigh* Not at all intuitive. Without @ONLY, quotes get escaped, but lose
their escape when passing through the lex/yacc parser.
tests: Test @ expansion with variable references in it

Yet another difference between string(CONFIGURE) and configure_file().
@ONLY makes sense, but string(CONFIGURE) will dereference the variable
as well while configure_file will not.
Merge topic 'variable-expansion-tests' into next

ae92a948 tests: Test @ expansion with variable references in it
075742e2 tests: Test string(CONFIGURE) expansion with escaped quotes
bc29b931 tests: Add variable expansion tests