Increase consistency in the missing argument check message, following
the ITK SW guide coding guidelines. Specifically, make the error message
be consistent.

Conform to ITK style guidelines in test ending message: add it where
missing and modify it where existing to match the ITK SWG module to
increase consistency.

Prefer declaring image dimensions as `constexpr <type>` over `struct` in
tests: a struct does not provide any particular advantage to host the
image dimensions.

Declare the type as an `unsigned int` and change the loop index
variables accordingly to avoid signed/unsiged comparison mismatch
warnings. Take advantage of the commit to limit the loop index
variables' scope to the loop.

Conform to ITK comment style for within code comments in tests: prefer
double forward slashes for within code comment blocks, and place the
comment blocks preferably immediately before the statemet they refer to.

Take advantage of the commit to use sentence cases in miscellaneous
comments.

Remove unnecessary and uninformative single-comment and empty comment
lines in tests.

Remove uninformative standard output message in `itkHistogramTest.cxx`.

Remove uninformative comments in tests.

Do not allow lonely `std::cout << std::endl` statements in miscellaneous
tests: move them to the preceding standard output message printing
statment.

Increase consistency in newline character use in tests: use `std::endl`
instead of `\n` to introduce new lines.

Remove `\n` characters in excess in miscellaneous output stream
messages.

Remove blank lines in excess in miscellaneous tests.