Migrated encoding testcases to new system

Testcases are now easier to build for encoding functions via the functions in
tests/encodings.c. This commit migrates old, encoding-specific testcases over
to the new location.
Migrated all NULL pointer tests for encoding functions

NULL pointer tests now moved out of per-encoding test files into the core tests/encoding.c file.
Migrated over invalid input tests for encodings

Moved these from per-encoding files to the core tests/encodings.c, and removed
the per-encoding files (which were now empty).