Fix flaky special char specs by not using `with_history` twice per test

There's something funny occasionally happening when `with_history` is
used twice in the same test. It seems to be happening more frequently
since asynchronous mode was enabled by default. My guess is it has
something to do with the `C-c` keys being sent toward the end not
consistently terminating the prompt. But I'm really not sure how it
would ever get into a `then` block like it seems to:

```
Failure/Error: wait_for { session.content }.to eq('echo "hello\nworld"')

  expected: "echo \"hello\\nworld\""
       got: "then> echo \"hello\\"
```

Sticking to only one `with_history` per terminal session (per test)
seems to fix the flakiness.

I also removed an old test case because I could not understand why it
was necessary and so couldn't write a good description for it. Could be
we'll need to add it back in at some point.

spec/strategies/special_characters_helper.rb

@@ -1,58 +1,71 @@
 shared_examples 'special characters' do
-  describe 'a special character in the buffer' do
-    it 'should be treated like any other character' do
+  describe 'a special character in the buffer should be treated like any other character' do
+    it 'asterisk' do
       with_history('echo "hello*"', 'echo "hello."') do
         session.send_string('echo "hello*')
         wait_for { session.content }.to eq('echo "hello*"')
       end
+    end
 
+    it 'question mark' do
       with_history('echo "hello?"', 'echo "hello."') do
         session.send_string('echo "hello?')
         wait_for { session.content }.to eq('echo "hello?"')
       end
+    end
 
+    it 'backslash' do
       with_history('echo "hello\nworld"') do
         session.send_string('echo "hello\\')
         wait_for { session.content }.to eq('echo "hello\nworld"')
       end
+    end
 
+    it 'double backslash' do
       with_history('echo "\\\\"') do
         session.send_string('echo "\\\\')
         wait_for { session.content }.to eq('echo "\\\\"')
       end
+    end
 
+    it 'tilde' do
       with_history('echo ~/foo') do
         session.send_string('echo ~')
         wait_for { session.content }.to eq('echo ~/foo')
       end
+    end
 
+    it 'parentheses' do
       with_history('echo "$(ls foo)"') do
         session.send_string('echo "$(')
         wait_for { session.content }.to eq('echo "$(ls foo)"')
       end
+    end
 
+    it 'square bracket' do
       with_history('echo "$history[123]"') do
         session.send_string('echo "$history[')
         wait_for { session.content }.to eq('echo "$history[123]"')
         session.send_string('123]')
         wait_for { session.content }.to eq('echo "$history[123]"')
       end
+    end
 
+    it 'octothorpe' do
       with_history('echo "#yolo"') do
         session.send_string('echo "#')
         wait_for { session.content }.to eq('echo "#yolo"')
       end
-
-      with_history('echo "#foo"', 'echo $#abc') do
-        session.send_string('echo "#')
-        wait_for { session.content }.to eq('echo "#foo"')
     end
 
+    it 'caret' do
       with_history('echo "^A"', 'echo "^B"') do
         session.send_string('echo "^A')
         wait_for { session.content }.to eq('echo "^A"')
       end
+    end
 
+    it 'dash' do
       with_history('-foo() {}') do
         session.send_string('-')
         wait_for { session.content }.to eq('-foo() {}')