atest: Converting files to .robot format (#1766).

Now converting .html files to .robot when possible.
atest: Converting test data files to use .robot extension (#1766)

Yet another, and hopefully last, massive commit. This time converting
HTML files to plain text/robot format when the idea is not to actually
test some HTML format related functionality. Only few files that
require a bit more work left after this.
atests: removed test suite which only tested functionality tested much more thoroughly elsewhere (e.g. in resource_and_variable_imports.robot and under test_libraries directory)
atest cleanup
removed test suite which was misleadingly named and contained only tests for functioanlity tested also elsewhere
atests: Continuing converting .html -> .robot (#1766).

Converted test_and_uk_settings.html -> .robot. This required some work
because this old file had tests that were already (partly) moved
elsewhere, invalid syntax on purpose, and so one.
atests: Found two more html resource files and converted them to .robot (#1766)
atest: Removed invalid_syntax.robot/html suite because it contained duplicate tests.

Initially planned to change test data file to use .robot extension
instead of .html (#1766). After looking at the file noticed that most
tests were duplicated with tests elsewhere. Those that were not dupes
were moved elsewhere and so the suite could be removed altogerher.
test fix after recent atest cleanup
genrunner: Use atest_resource.robot, not txt. Also check input path.
report/log: Fix showing HTML special chars in statistic table pop-up.

Fixes #1767.
Process: 1) Own test suite for env config, 2) Better error if using invalid kwargs.
Process tests: Renamed resource file.
Process tests: Separate suite for passing arguments to processes and some other cleanup.
Process: Fixed newly added tests for passing arguments that didn't work on Windows. Also enhanced them otherwise.
Process tests: Workaround for some weird IronPython bug/feature.
Process: Convert process arguments to strings automatically.

Fixes #1738.
XML atests: Renamed resource.robot to more explicit xml_resource.robot
XML: New keywords to set/remove tag/text/attr of multiple elements.

Resolves #1749.
Tuning project README.