Tests: Add two RISC-V Filter test files.

These test files achieve 100% code coverage in
src/liblzma/simple/riscv.c. They contain all of the instructions that
should be filtered and a few cases that should not.

tests/files/README

@@ -80,6 +80,14 @@
     good-1-arm64-lzma2-2.xz is like good-1-arm64-lzma2-1.xz but with
     non-zero start offset. XZ Embedded doesn't support this file.
 
+    good-1-riscv-lzma2-1.xz uses the RISC-V filter and LZMA2. The
+    uncompressed data is constructed so it tests all of the instructions
+    that should be encoded and a few that should not. Additionally, the
+    file contains random bytes to help test unforeseen corner cases.
+
+    good-1-riscv-lzma2-2.xz is like good-1-riscv-lzma2-1.xz but with
+    non-zero start offset. XZ Embedded doesn't support this file.
+
     good-1-lzma2-1.xz has two LZMA2 chunks, of which the second sets
     new properties.