Prior to this commit, the parameterized DataBufferFactory was never
actually used when setting up the WebClient for each test. This was due
to an oversight when migrating from JUnit 4 to JUnit Jupiter.
See: https://github.com/reactor/reactor-netty/issues/860
This commit fixes this by converting the existing @BeforeEach method to
a local setup method that is invoked from each
@ParameterizedDataBufferAllocatingTest method.
In addition, in order to avoid the 2 second "quiet period" that is
incurred when destroying the ReactorResourceFactory, this commit moves
the setup and destruction of the ReactorResourceFactory to new
@BeforeAll and @AfterAll methods.
The test instance lifecycle has also been switched to PER_CLASS to avoid
static state in the test class.
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