The tests weren't being run.  Total tests increase from 2742 to 2769.

Also fix 'when using a class extending String' test.
It had been failing with...
```
RuntimeError:
       can't modify frozen AlsoString
```

version 4.20.0 mergeback

Since the purpose of this validation type is to check if the given parameter is a valid UNIX path it should also cover the root path "slash" since it might be possible that it is specified as a target directory.

I used the example code from this README file but i got downvotes from CI / jenkins / puppet-lint because it doesn't conform to Puppet style guides to use double quotes here. It produces "double quoted string containing no variables" warnings.

I wanted to fix the example locally but it was pointed out to me we should fix it upstream stdlib instead.

Fix filenames of two function spec tests

(maint) modulesync 892c4cf

removing duplicate test absolute_path test

Allow root as valid UNIX path

Based on a Stack Overflow question, it was noted that the documentation
for `file_line` isn't complete and the underlying behaviour somewhat
confusing.

https://stackoverflow.com/questions/46468922/how-to-change-the-contents-of-a-file-by-using-puppet/46473458

In this patch I add additional unit tests and better examples and
complete documentation.

(maint) Clarify docs and add new tests

https://tickets.puppetlabs.com/browse/MODULES-5003 gave rise to
#788 and
#794 which caused
different behavior based on whether the line value was matched by the
match regex or not.

The change in behavior was both breaking and broken, though that was
hard to tell because the behavior was ill-described in general.

[bugfix] This commit resolves the breaking behavior by reverting the
behavior of "replacing matches when a line matching `line` exists even
when `multiple` is set to `true`".

[feature] This commit adds a new parameter to make file_line replace all
matches universally with the `line` value, even when the line exists
elsewhere in the file. This feature only affects modifying multiple
lines in a file when the `line` value already exists.

[bugfix] This commit more strictly defines the various interactions of
`ensure`, `match`, `append_on_no_match`, `replace`, `multiple`, and
`replace_all_matches_not_matching_line`. It also more clearly documents
and tests these interactions.

(MODULES-5651) Do not append infinitely

(MODULES-5806) release prep for version 4.21.0

README fixups for 4.21.0 release