Previously, passing 0 for newlen would not truncate the string at all.

This adds handling of this case, freeing the old string and creating a new empty string.

…XXXReply functions

 * adds TEST.STRING.TRUNCATE and includes it in TEST.IT

   This test fails unless issue redis#3717 is resolved

 * add handling for ERROR replies in TestAssertStringReply and TestAssertIntegerReply

   Previously these TestAssert functions would output something like,
    `<test> Unexpected reply type 1`
   rather than emitting the error message of what was wrong.

 * add `test-module` make target

 * `make distclean` now cleans module src/test directories

 * `runtest-moduleapi` now builds and exercises `testmodule`

 * add `clean` target for `commandfilter`

 * add `.gitignore` for `commandfilter`

 * typo "SOME TEST NOT PASSED!" -> "SOME TEST DID NOT PASS!"

…wLenZero

 Conflicts:
	runtest-moduleapi
	src/Makefile
	tests/modules/Makefile