fix(test/dumplog): tostring(rv) before formatting as string

For example, implicitly converting a table to a string works in LuaJIT,
but needs to be done explicitly with tostring() in Lua 5.1.

This can cause issues when testing a non-JIT build if eq(), for example,
fails with a table argument. E.g: eq({}, {1}) will not print the details
of the assertion failure, but will instead print a less helpful "string
expected, got table" error.

test/helpers.lua

@@ -70,7 +70,7 @@ local function dumplog(logfile, fn, ...)
   if status == false then
     logfile = logfile or os.getenv('NVIM_LOG_FILE') or '.nvimlog'
     local logtail = module.read_nvim_log(logfile)
-    error(string.format('%s\n%s', rv, logtail))
+    error(string.format('%s\n%s', tostring(rv), logtail))
   end
 end
 function module.eq(expected, actual, context, logfile)