Add specs of snippets from docs

spec/docs/subject_spec.cr

@@ -0,0 +1,32 @@
+require "../spec_helper"
+
+Spectator.describe "subject" do
+  subject(array1) { [1, 2, 3] }
+  subject(array2) { [4, 5, 6] }
+
+  it "has different elements" do
+    expect(array1).to_not eq(subject) # array2 would also work here.
+  end
+
+  let(string) { "foobar" }
+
+  it "isn't empty" do
+    expect(string.empty?).to be_false
+  end
+
+  it "is six characters" do
+    expect(string.size).to eq(6)
+  end
+
+  let(array) { [0, 1, 2] }
+
+  it "modifies the array" do
+    array[0] = 42
+    expect(array).to eq([42, 1, 2])
+  end
+
+  it "doesn't carry across tests" do
+    array[1] = 777
+    expect(array).to eq([0, 777, 2])
+  end
+end