Drop the TESTS edge kind and its filename-affix machinery

[janrito]

Remove the TESTS edge — the graph link from a test to the code it
exercises — and the per-plugin configuration that drove it.

infer_test_edges linked a test function to the source symbol it tests.
It located the source by filename convention — test_prefix /
test_suffix stripped test_foo.py down to foo to guess the matching
file — then either linked test functions to the symbols the test actually
imports, or, failing that, fell back to linking every test function to the
first symbol in the file. That fallback is a guess, and the rest is
redundant: the “test imports the code” relationship it leaned on is already
recorded as an IMPORTS edge, independently. So the dedicated TESTS edge
was a noisy, name-matching-dependent layer over a signal the import edge
already carries, and test_prefix / test_suffix were per-plugin
configuration that every one of the ~20 languages still to be packaged
would otherwise have to carry.

Import edges are untouched — including cross-language ones, like an HTML
page's <script> and <link> resolving to js/ts/css, and language
injection — so only the test-specific edge and its configuration go.
Confirming that a language actually produces its edges, the one thing the
removed feature was informally used for, is covered instead by each
package's sample, which pins its real import edges; injection-capable
packages embed other-language code so their cross-language edges are
pinned too.

The docs that advertised the feature — the find-refs edge-kind lists and
the rbtr_find_refs tool description — drop their tests mentions.

No extraction changes. The generated TypeScript EdgeKind type drops
"tests" to match.