tests: Support limiting the testcases to a particular module.
tests: Support limiting the testcases to a particular module.

For example:
 make && make -C tests check-bson

This command would only run the BSON test cases. Appending -unit or
-func will further limit it to unit or functional tests.

algernon/libmongo-client,algernon/libmongo-client,algernon/libmongo-client