fix: Guard against nan in test stat calculation

[matthewfeickert]

Description

• Resolves AsymptoticCalculator.teststatistic issues warning and returns nan when qmuA == 0 #1992
• Resolves single bin fit with s = 0.0, b = 1.9, deltab = 0.4 gives NaN #529
• Resolves Fix documentation for qmu_tilde test statistic #1994

Checklist Before Requesting Reviewer

• Tests are passing
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

Before Merging

For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

* Guard against nan from division by zero in pyhf.infer.calculators.AsymptoticCalculator.teststatistic.
* Add use of 'less than or equal to' to docs for tests stats to match equations 14
  and 16 of https://arxiv.org/abs/1007.1727.
* Add tests to ensure that nan conditions in Issue #529 and Issue #1992 are not possible.

[matthewfeickert]

@masonproffitt I'll bring this out of draft in the morning my time, but I'm about to go to bed.

This needs:

• Verification and agreement across the devs.
• To have a test that will ensure that a nan isn't created.

[codecov]

Codecov Report

Base: 98.28% // Head: 98.28% // No change to project coverage 👍

Coverage data is based on head (9d6e7f9) compared to base (287bfae).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1993   +/-   ##
=======================================
  Coverage   98.28%   98.28%           
=======================================
  Files          68       68           
  Lines        4479     4479           
  Branches      730      730           
=======================================
  Hits         4402     4402           
  Misses         45       45           
  Partials       32       32           

Flag	Coverage Δ
contrib	27.61% <ø> (ø)
doctest	61.30% <ø> (+0.04%)	⬆️
unittests-3.10	96.22% <ø> (ø)
unittests-3.7	96.20% <ø> (ø)
unittests-3.8	96.24% <ø> (ø)
unittests-3.9	96.27% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/pyhf/infer/calculators.py	100.00% <ø> (ø)
src/pyhf/infer/test_statistics.py	100.00% <ø> (ø)

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