tests/lapi: add string.buffer tests

[ligurio]

The patch add tests for LuaJIT's string buffer library.

1. https://luajit.org/ext_buffer.html

Blocked by ligurio/luzer#62

[Buristan]

Hi, Sergey!
Thanks for the patch!
Please consider my comments below.

[ligurio]

Also, updated buffer_putcdata:

--- a/tests/lapi/string_buffer_torture_test.lua
+++ b/tests/lapi/string_buffer_torture_test.lua
@@ -74,14 +74,16 @@ local function buffer_putf(self)
     self.buf:putf("%s", str)
 end
 
--- Appends the given len number of bytes from the memory pointed
+-- Appends the given `len` number of bytes from the memory pointed
 -- to by the FFI cdata object to the buffer. The object needs to
 -- be convertible to a (constant) pointer.
 -- Usage: buf = buf:putcdata(cdata, len)
 local function buffer_putcdata(self)
     local n = self.fdp:consume_integer(1, test_lib.MAX_INT)
     local cdata = ffi.new("uint8_t", n)
-    self.buf:putcdata(cdata, ffi.sizeof(cdata))
+    -- The function may throw an error "cannot convert
+    -- 'unsigned char' to 'const void *'".
+    pcall(self.buf.putcdata, self.buf, cdata, ffi.sizeof(cdata))
 end
 
 -- This method allows zero-copy consumption of a string or an FFI

[Buristan]

Sergey,
Thanks for the fixes!
LGTM, after resolving the discussions above.