x/tools/go/packages/packagestest: support markers in go.mod files

[stamblerre]

Users of packagestest may want to put markers in go.mod files to run tests. This is particularly relevant for gopls handling go.mod files correctly.

/cc @ridersofrohan @ianthehat

[ridersofrohan]

In addition, this would be helpful for refactoring internal/lsp/mod/mod_test.go to use x/tools/go/packages/packagestest.

[gopherbot]

Change https://golang.org/cl/216838 mentions this issue: go/expect: add marker support for go.mod files

[gopherbot]

Change https://golang.org/cl/216839 mentions this issue: go/packages/packagestest: do not overwrite existing go.mod file

[gopherbot]

Change https://golang.org/cl/217541 mentions this issue: internal/lsp/tests: standardize testdata folder format

[gopherbot]

Change https://golang.org/cl/218502 mentions this issue: go/packages/packagestest: fix grouping to account for module versions