tests/lapi: fix require string.buffer

The patch add a check for presence of string.buffer module to the
tests. The module string.buffer built into LuaJIT, but not
available in Tarantool and before the patch the tests were failed.

Needed for #google/oss-fuzz#14656

Author: ligurio

tests/lapi/string_buffer_encode_test.lua

@@ -27,7 +27,11 @@ if test_lib.lua_version() ~= "LuaJIT" then
     os.exit(0)
 end
 
-local string_buf = require("string.buffer")
+local has_string_buf, string_buf = pcall(require, "string.buffer")
+if not has_string_buf then
+    print("Unsupported version.")
+    os.exit(0)
+end
 
 local function TestOneInput(buf, _size)
     local fdp = luzer.FuzzedDataProvider(buf)

tests/lapi/string_buffer_torture_test.lua

@@ -37,7 +37,11 @@ if test_lib.lua_version() ~= "LuaJIT" then
 end
 
 local ffi = require("ffi")
-local string_buf = require("string.buffer")
+local has_string_buf, string_buf = pcall(require, "string.buffer")
+if not has_string_buf then
+    print("Unsupported version.")
+    os.exit(0)
+end
 local unpack = unpack or table.unpack
 
 local MAX_N = 1e2