extension/src/goTest: skip testdata directories in test discovery

[ChrisJr404]

The Test Explorer walks the workspace to discover _test.go files, but it
descended into testdata directories too. The go command treats testdata
as inert, so tests defined there aren't real and shouldn't appear in the
tree.

walk now skips directories named testdata, which keeps both walkWorkspaces
and walkPackages from descending into them. That covers discovery through
the tree, but a user can still open a _test.go file that lives under a
testdata directory directly, so processDocument returns early when the
document's path contains a testdata segment. This follows the approach
firelizzard18 outlined on the issue.

Added resolver tests for both paths: a testdata package is skipped during
module resolution, and a file opened from within testdata resolves no
tests.

Fixes #1854

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