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