Allow testSendToGetLargeContent peak memory usage to be specified externally

[phil-davis]

See discussion sabre-io/dav#1409 (comment)

The 'sabre/http' PHP unit tests are also run in the sabre/dav CI. The PHP environment is a bit "bigger" there, and memory_get_peak_usage() returns about 70MB, above the 60MB limit that the test allows.

This PR adds an env variable that can be defined to specify the peak memory usage that is allowed for the test. That will let us be more flexible in other CI.

I called it SABRE_HTTP_TEST_GET_LARGE_CONTENT_MAX_PEAK_MEMORY_USAGE - naming is hard, and I wanted a name that won't conflict with anything else in anyone's environment!

[codecov]

Codecov Report

Merging #187 (87fbcab) into master (87fbcab) will not change coverage.
The diff coverage is n/a.

❗ Current head 87fbcab differs from pull request most recent head 94321c1. Consider uploading reports for the commit 94321c1 to get more accurate results

@@            Coverage Diff            @@
##             master     #187   +/-   ##
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  Coverage     94.11%   94.11%           
  Complexity      260      260           
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  Files            15       15           
  Lines           884      884           
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  Hits            832      832           
  Misses           52       52           

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[phil-davis]

And I think that I will have to make a patch release so that sabre/dav CI will find and use this test change. That's a little bit sad for the real consumers of this repo - the change is test-only and actually does not fix or alter any real behavior.