Use lexical-binding in most src tests

Use lexical-binding in most src tests

* test/src/charset-tests.el:
* test/src/chartab-tests.el:
* test/src/cmds-tests.el:
* test/src/coding-tests.el (top-level)
(generate-ascii-file, generate-mostly-nonascii-file):
* test/src/doc-tests.el:
* test/src/floatfns-tests.el:
* test/src/font-tests.el:
* test/src/keymap-tests.el:
* test/src/process-tests.el (top-level)
(process-test-sentinel-wait-function-working-p)
(process-test-stderr-buffer, process-test-stderr-filter):
* test/src/textprop-tests.el:
* test/src/thread-tests.el:
* test/src/timefns-tests.el:
* test/src/undo-tests.el:
* test/src/xml-tests.el: Use lexical-binding.