Add forward compat tests for macros

[nicolasstucki]

No description provided.

[prolativ]

For _c suffix you should specify the full version of the compiler, e.g. _c3.0.2.
Patch version is skipped only for _r, e.g. _r3.0.

[nicolasstucki]

tests/pos-macros/backwardCompat-3.1 is failing and I can't figure out why because the error is not shown (see #14303).

[nicolasstucki]

I reproduced it manually and got

error while loading Macros$,
class file out2/Macros.class is broken, reading aborted with class dotty.tools.tasty.UnpickleException
TASTy signature has wrong version.
 expected: {majorVersion: 28, minorVersion: 1}
 found   : {majorVersion: 28, minorVersion: 2 [unstable release: 1]}

This TASTy file was produced by an unstable release.
To read this TASTy file, your tooling must be at the same version.
The TASTy file was produced by Scala 3.1.2-RC1-bin-SNAPSHOT-nonbootstrapped-git-c3eaadd.

Minimized in #14306

[prolativ]

LGTM in general but shouldn't we actually swap the naming of forward/backward-compat tests? Generally when we test forward binary/TASTy compatibility we should check that the compilation output from a newer compiler can be consumed by an older compiler (which is the case if we add no compiler version suffix to the first file but we specify the older compiler for the second file)