Go: Use `toolchain` directives for version selection if available, and add tests

[mbg]

Context

In Go 1.21, the Go team started making a distinction between language and toolchain versions. Historically, the Go version is declared with a go directive in a go.mod file, or a go.work file. Since Go 1.21, go directives are used to declare the language version. A new toolchain directive may be used to explicitly declare the toolchain version. For backwards compatibility, if there is no toolchain directive, the language version is used as the toolchain version. In the Go autobuilder, we have numerous places where we try to determine the "version" of Go that is in use, should be installed, etc. Here, we are mainly interested in the toolchain version.

What this PR addresses

So far, we have mainly been looking at go directives in go.mod files, and recently go.work files, for this. However, if a toolchain directive is present in either type of file, then this determines the toolchain version. We have not been considering this and this PR addresses that shortcoming by modifying the autobuilder to check for the presence of toolchain directives when determining the version that is in use.

This PR also adds a number of tests for the functions involved in this process.

How to review

Best reviewed commit-by-commit.