lib/tinytest: Rename usage() to tinytest_usage().
lib/tinytest: Rename usage() to tinytest_usage().

While the function is static, some ports use tinytest by just #including
the source file into another source file. So, add namespace prefix to
this function to avoid clashes. (Affected unix port.)

pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython