cmd/go: fix tests for new builder environment

Fix two long tests that fail in the builders we're trying out:

- TestQueryImport was failing with:
  open /nonexist-gopath/pkg/sumdb/sum.golang.org/latest: no such file or directory
  which eventually turns out to be because it couldn't create
  /nonexist-gopath because it wasn't running as root. The test already
  uses a temporary GOPATH, but missed overriding a configuration
  variable set at init time.
- test_flags fails if the working directory has /x/ in it, which it now
  happens to.

Change-Id: Ideef0f318157b42987539e3a20f9fba6a3d3bdd0
Reviewed-on: https://go-review.googlesource.com/c/go/+/480255
Run-TryBot: Heschi Kreinick <heschi@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/cmd/go/internal/modload/query_test.go

@@ -55,6 +55,7 @@ func testMain(m *testing.M) (err error) {
 	os.Setenv("GOPATH", dir)
 	cfg.BuildContext.GOPATH = dir
 	cfg.GOMODCACHE = filepath.Join(dir, "pkg/mod")
+	cfg.SumdbDir = filepath.Join(dir, "pkg/sumdb")
 	m.Run()
 	return nil
 }

src/cmd/go/testdata/script/test_flags.txt

@@ -15,8 +15,7 @@ stdout '\Aok\s+example.com/x\s+[0-9.s]+\n\z'
 # Even though ./x looks like a package path, the real package should be
 # the implicit '.'.
 ! go test --answer=42 ./x
-stderr '^no Go files in .+$'
-! stderr '/x'
+stderr '^no Go files in '$PWD'$'
 
 # However, *flags* that appear after unrecognized flags should still be
 # interpreted as flags, under the (possibly-erroneous) assumption that