Fixes zsh-users/zsh-syntax-highlighting#182.
Prerequisite for testing issue #228.
* tests/test-highlighting.zsh
(run_test): Move functionality to run_test_internal; make run_test be a wrapper
that handles creating and cleaning up the tempdir.
* tests/README.md: Document the new feature.
* "highlighters/main/test-data/path-space- .zsh"
* highlighters/main/test-data/path-tilde-named.zsh
* highlighters/main/test-data/path.zsh
Change test data to not depend on being run from the source directory.
All these files should be sourced, not executed; and Debian's lintian complains:
W: zsh-syntax-highlighting: script-not-executable usr/share/zsh-syntax-highlighting/zsh-syntax-highlighting.zsh
