Make MySQL ftest use shared fixture

[artem-shelkovnikov]

Part of https://github.com/elastic/enterprise-search-team/issues/3397

This PR changes the fixture for the respected content source to make use of shared test setup using WeightedFakeProvider class.

This class takes care of generating large fake data with certain distribution, for example:

# In 65% cases generate small files
# In 20% cases generate medium size files
# In 10% cases generate large files
# in 5% cases generate huge files
fake_provider = WeightedFakeProvider(weights=[0.65, 0.2, 0.1, 0.05])

fake_provider.get_html() # <---- gets HTML of size depending on distribution
fake_provider.get_text() # <---- gets text of size depending on distribution

# Important difference
# get_text() returns huge amount of text that can be ingested, for example 2MB payload
# get_html() returns a huge payload that has too little text, for example for 2MB html it's around 1KB of text to text download/subextraction of rich content better

The final goal of the PR is to have more comparable benchmarks in our nightly functional tests of the amount of memory or cpu that is needed to run the connector.

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