Tests/RunCMake: Document stripping of expected output

Document that trailing newlines are stripped from the -stdout.txt
and -stderr.txt content before using as a regular expression.
export: Error when exporting a target without a language

First, it prevents a NULL dereference and second it reiterates that
targets without languages are not supported by CMake.

Add a RunCMake.ExportWithoutLanguage test exporting a library without a
languages.
Merge topic 'doc-RunCMake-strips-expected'

c80a4f9 Tests/RunCMake: Document stripping of expected output
Merge topic 'dev/export-target-without-language'

5837f19 export: Error when exporting a target without a language