tests/lowlevel: Only compare stdout, ignore stderr.
tests/lowlevel: Only compare stdout, ignore stderr.

The goal of the test is to compare stdout to make sure it equals to
what is expected.

Ignore stderr as there may be unrelated warnings there, for example the
warning that GopherJS prints when source map support module is not
installed.

Fixes the following invalid test failure:

	go test -run=TestTimeInternalizationExternalization github.com/gopherjs/gopherjs/tests
	--- FAIL: TestTimeInternalizationExternalization (0.30s)
		lowlevel_test.go:29: got != want:
			got:
			gopherjs: Source maps disabled. Use Node.js 4.x with source-map-support module for nice stack traces.
			{ S: 'externalization ok' }
			Sat, 29 Aug 2015 20:56:00 GMT

			want:
			{ S: 'externalization ok' }
			Sat, 29 Aug 2015 20:56:00 GMT
	FAIL
	FAIL	github.com/gopherjs/gopherjs/tests	0.313s

gopherjs/gopherjs,albrow/gopherjs,gopherjs/gopherjs,albrow/gopherjs,albrow/gopherjs,gopherjs/gopherjs,gopherjs/gopherjs,gopherjs/gopherjs