M15: tag linalg describe blocks for selective test runs

Add @describetag :linalg to all linalg describe blocks so they
can be included/excluded via mix test --only/--exclude linalg.

Author: ausimian

test/emily/backend_fallbacks_test.exs

@@ -145,6 +145,7 @@ defmodule Emily.Backend.FallbacksTest do
   end
 
   describe "native linalg" do
+    @describetag :linalg
     test "lu returns (p, l, u) and reconstructs input" do
       t = emily([[2.0, 1.0], [1.0, 3.0]])
       {p, l, u} = Nx.LinAlg.lu(t)

test/emily/backend_test.exs

@@ -674,6 +674,7 @@ defmodule Emily.BackendTest do
   # ---------------- Linalg ----------------
 
   describe "linalg" do
+    @describetag :linalg
     property "lu: P * L * U ≈ A for random well-conditioned matrices" do
       check all(a <- square_matrix(), max_runs: @max_runs) do
         a_safe = make_well_conditioned(a)

test/emily/compiler_test.exs

@@ -130,6 +130,7 @@ defmodule Emily.CompilerTest do
   end
 
   describe "linalg" do
+    @describetag :linalg
     test "dot product (matmul)" do
       a = Nx.iota({3, 4}, type: :f32)
       b = Nx.iota({4, 2}, type: :f32)

test/emily/native_test.exs

@@ -453,6 +453,7 @@ defmodule Emily.NativeTest do
   # ---------- Linalg ----------
 
   describe "linalg" do
+    @describetag :linalg
     test "matmul: 2x3 @ 3x2" do
       a = f32([1.0, 2.0, 3.0, 4.0, 5.0, 6.0], [2, 3])
       b = f32([7.0, 8.0, 9.0, 10.0, 11.0, 12.0], [3, 2])