modules: Add newlen == 0 handling to RM_StringTruncate (#3717) (#3718)

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.

Other changes:
- Move `src/modules/testmodule.c` to `tests/modules/basics.c`
- Introduce that basic test into the test suite
- Add tests to cover StringTruncate
- Add `test-modules` build target for the main makefile
- Extend `distclean` build target to clean modules too

tests/unit/moduleapi/basics.tcl

@@ -0,0 +1,13 @@
+set testmodule [file normalize tests/modules/basics.so]
+
+
+# TEST.CTXFLAGS requires RDB to be disabled, so override save file
+start_server {tags {"modules"} overrides {save ""}} {
+    r module load $testmodule
+
+    test {test module api basics} {
+        r test.basics
+    } {ALL TESTS PASSED}
+
+    r module unload test
+}