Follow up to #2104 and #2105. This ensures that `Purchases` is re-created for each test.

…2105)

Follow up to #2104. See also #2101. This has uncovered a huge source of
leaks across many tests. Fixing this provides many benefits:

- Faster test runs
- Lower memory overhead
- Isolated state across each test run
- Ensure that these types don't leak outside of tests too

While working on #2533, I knew that my proof of concept had a retain cycle. I however was very confused when it wasn't detected by this mechanism introduced first introduced in #2104.

Turns out that this had been wrong the whole time: the current implementation, as explained by the comment, was executing those blocks in LIFO order, which meant that the references were being set to `nil` before the assertions could be created.
This meant that by the time the `except { [weak purchases = self.purchases] ... }` lines were called, `self.purchases` itself was `nil`, so nothing was being checked.

This simplifies the implementation by inlining the checks, and saving a `weak` reference before clearing the singleton and all the local references.

Luckily there was only one failing test that had to do with how `Purchases` was being created, and implementation details of `expectFatalError`. To avoid this, I changed the test to use the `Purchases.init` method directly.

While working on #2533, I knew that my proof of concept had a retain
cycle. I however was very confused when it wasn't detected by this
mechanism introduced first introduced in #2104.

Turns out that this had been wrong the whole time: the current
implementation, as explained by the comment, was executing those blocks
in LIFO order, which meant that the references were being set to `nil`
before the assertions could be created.
This meant that by the time the `except { [weak purchases =
self.purchases] ... }` lines were called, `self.purchases` itself was
`nil`, so nothing was being checked.

This simplifies the implementation by inlining the checks, and saving a
`weak` reference before clearing the singleton and all the local
references.

Luckily there was only one failing test that had to do with how
`Purchases` was being created, and implementation details of
`expectFatalError`. To avoid this, I changed the test to use the
`Purchases.init` method directly.