tests: replace hard-coded `/dev/null` with variable

- add variable for the null device filename and use that in Perl code.
- initialize this variable with `NUL` on native Windows.
-

tests: replace hard-coded `/dev/null` with variable

- add variable for the null device filename and use that in Perl code.
- initialize this variable with `NUL` on native Windows.
- add `%DEV_NULL` variable and use it in tests.

Fixes `The system cannot find the path specified.` messages seen when
running `runtests.pl` with native Windows Perl.

Also adjust code to not break mcedit syntax highlighting.

Cherry-picked from #14949
Closes #15098

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misc: fix spelling mistakes

Reported-by: musvaage on github
Fixes #11171
Closes #11172

runtests: support buffering log messages in runner & servers

Log messages generated with logmsg can now be buffered and returned from
the runner as a return value. This will be needed w

runtests: support buffering log messages in runner & servers

Log messages generated with logmsg can now be buffered and returned from
the runner as a return value. This will be needed with parallel testing
to allow all messages for one test to be displayed together instead of
interspersed with messages of multiple tests. Buffering can be disabled
by setting a logging callback function with setlogfunc, which is
currently being done to preserve existing logging behaviour for now.

Some additional output is generated in verbose and debugprotocol modes,
which don't always use logmsg. These modes also impact some servers
which generate extra messages. No attempt is made to buffer everything
if these modes are enabled.

Ref: #10818
Closes #11016

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runtests: tightened servers package exports

The defaults are intended for runtests.pl, whereas runner.pm needs to
explicitly specify them.

devtest: add a new script for testing the test harness

This is currently useful for starting a test server on its own without
an associated test, which can be used for interactive curl t

devtest: add a new script for testing the test harness

This is currently useful for starting a test server on its own without
an associated test, which can be used for interactive curl testing or
for validating parts of the test harness itself. More commands can be
added to perform additional functions in the future.

Ref: #10818
Closes #11008

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