test: adjust lua-Harness test error assertion

The current pattern in 303-package.t for the test case loading invalid
Lua script is too tight for the following reasons revealed by Tarantool
testing:
* The platform might prepend the common prefix to all Lua errors, so
  anchoring the pattern to the beginning of the error message is too
  strict for matching the particular errors.
* The platform might canonicalize module names (i.e., use absolute
  paths) for searchers and loaders, so matching the current working
  directory via "./" is too straight-forward.

Hence, this patch removes the starting anchor from the pattern and fixes
the last part of it to match only the "basename" of the module being
loaded. As a result, the check is not too strict and Lua-specific but
rather starts following common sense.

test/lua-Harness-tests/303-package.t

@@ -199,7 +199,7 @@ return complex
     f:write [[?syntax error?]]
     f:close()
     error_matches(function () require('syntax') end,
-            "^error loading module 'syntax' from file '%.[/\\]syntax%.lua':",
+            "error loading module 'syntax' from file '.+[/\\]syntax%.lua':",
             "function require (syntax error)")
     os.remove('syntax.lua') -- clean up