cmd/govulncheck: rearrange test files

Though this is slightly less organized (no longer dividing certain test
dirs into spearate json/text directories), it fixes a bug where those
tests weren't being ran. This is because `cmdtest.Read` globs on files
matching the pattern *.ct, meaning it doesn't search recursively for .ct
files. It also doesn't allow appending more tests to the test suite, so
it isn't possible to scan both a single directory wildcard (dir/*) and
the multiple directory case (dir/*/*).

This isn't the most ideal change, but all JSON tests end with _json.ct,
so they're still relatively human readable.

Change-Id: Ib0d55a3591535c3bb973af391359b1071722c4a0
Reviewed-on: https://go-review.googlesource.com/c/vuln/+/545635
Reviewed-by: Zvonimir Pavlinovic <zpavlinovic@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: Maceo Thompson