org-element: Use stricter regexps for boundaries of elements
* lisp/org-element.el (org-element-center-block-parser):
(org-element-drawer-parser, org-element-dynamic-block-parser,
org-element-example-block-parser, org-element-export-block-parser,
org-element-latex-environment-parser, org-element-paragraph-parser,
org-element-property-drawer-parser, org-element-src-block-parser,
org-element-verse-block-parser): Use stricter regexps for boundaries
of elements.
* testing/lisp/test-org.el: Fix a test.