[algorithms][is_valid] add variant support
[algorithms][is_simple] add variant support
[test][algorithms][is_valid] fix order of booleans in BOOST_CHECK_MESSAGE;
add test for variant support;
[test][algorithms][is_simple] remove obsolete includes;
put WKTs of test cases in a single line;
use BOOST_CHECK_MESSAGE instead of BOOST_CHECK;
polish some variable names;
add test for variant support;