[feature] Support [test_requires] in conanfile.txt in Conan v2 (and v1?)

[leha-bot]

I think that we should add support for [test_requires] into the conanfile.txt for uniformity with conanfile.py's self.test_requires().

• I've read the CONTRIBUTING guide.

[memsharded]

test_requires in conanfile.txt are basically exactly the same as requires, as the only difference is "visibility" and that doesn't affect if there are no downstream consumers, and by definition, a conanfile.txt cannot have consumers because it cannot create a pattern.

Asking @czoido for further feedback, if we should add this as syntactic sugar

[memsharded]

Implemented in #12699, for 2.0