gotest: force test package to be imported first

gotest: force test package to be imported first

    When compiling the x_test package, force the test package to be
    imported first.  That ensures that we will see the types defined in
    the test package before the types defined in the non-test version of
    the package.  This matters if the types differ in some way, such as by
    adding a new method.

    This avoids a failure in internal/poll on Solaris, in which the test
    package adds a method to a type (FD.EOFError).  I think it was Solaris-
    specific because files are sorted in a different order by default.

    The go tool handles this kind of thing correctly, by rebuilding
    dependent packages.  This is just a hack sufficient to run the libgo
    testsuite without using the go tool.

    Fixes https://gcc.gnu.org/PR91712

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/194637

From-SVN: r275648