cmd/compile/internal/syntax: TestStdLib verbosely broken on Windows [1.13 backport]

[gopherbot]

@dmitshur requested issue #35175 to be considered for backport to the next 1.13 minor release.

@gopherbot, please consider this issue for backport to Go 1.13. This is a test fix. It is needed to resolve a test failure in the cmd/compile/internal/syntax package on windows-amd64-longtest builder, which could be masking other problems (due to #36181) and making releasing Go 1.13.x more difficult.

[gopherbot]

Change https://golang.org/cl/223703 mentions this issue: [release-branch.go1.13] cmd/compile/internal/syntax: don't hardwire path separator in test

[gopherbot]

Closed by merging 5a31a97 to release-branch.go1.13.