Remove the need for out-of-source build trees with non-Unity Ninja builds

[claremacrae]

In a project where I'm using ApprovalTests, and I'm out-of-source builds so that Ninja CI builds should work, I'm finding that the compile-time error check is being triggered...

https://github.com/approvals/ApprovalTests.cpp/blob/master/doc/TroubleshootingMisconfiguredBuild.md#compilation-error

... I expected this Ninja build to work, because I'm doing out-of-source builds...

It's happened in one project where I'm loading the ApprovalTests.cpp project via CMake's FetchContent - so even though the build tree is outside the source directory, FetchContent is putting ApprovalTests inside the build-tree - which I think is resulting in relative paths...

This is another situation:

claremacrae/ApprovalTests.cpp.CMakeSamples#5

[claremacrae]

Because of this issue, I've experiment with adding some code to ApprovalTests::TestName::setFileName() so that, if it finds that the supplied source file does not exist, it tries looking in the parent dir, the grandparent dir, and a few more....

And if it finds the source file there, it remembers the amount of "../" that it needed to add, and uses that in the remaining tests in that test suite run.

The code is a bit ugly at the moment - but it's a big improvement in behaviour, as we would be able to get rid of most or all of this Troubleshooting page, for newer releases: https://github.com/approvals/ApprovalTests.cpp/blob/master/doc/TroubleshootingMisconfiguredBuild.md

.... and we would also get rid of the compile-time check as to whether the build was configured correctly...

[claremacrae]

The code so far is here - it runs on all CI platforms...

a11ccaa...a2b0081

Things that definitely need to be done:

• Use SystemUtils::getDirectorySeparator()
• Tidy up the code
• Add some tests
• Remove the compile-time check: CheckFileMacroIsAbsolute.h
• Decide how to update the docs TroubleshootingMisconfiguredBuild.md

[claremacrae]

This is fixed in the soon-to-be-released v.10.8.0