refactor: move runtime tests into a separate file

[pugachAG]

This PR moves NightshadeRuntime tests module into a separate file without changing anything else.

See #10275 for more context.

In case having tests as a separate file is a good idea for the following reasons:

• these are mostly integration-style tests with heavy setup (see TestEnv) and quite a lot of code, so to me it seems like it deserves to be in a separate file
• the code is already in mod.rs file, so we don't need to create a separate directory structure.

[akhi3030]

Disappointed in github's UI to not provide a more sensible diff.

Can you provide a brief justification for the change please?

[codecov]

Codecov Report

Attention: 90 lines in your changes are missing coverage. Please review.

Comparison is base (90355d4) 71.78% compared to head (e84dd94) 71.76%.

Files	Patch %	Lines
nearcore/src/runtime/tests.rs	92.96%	84 Missing and 6 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10274      +/-   ##
==========================================
- Coverage   71.78%   71.76%   -0.02%     
==========================================
  Files         711      712       +1     
  Lines      142936   142874      -62     
  Branches   142936   142874      -62     
==========================================
- Hits       102613   102540      -73     
- Misses      35594    35604      +10     
- Partials     4729     4730       +1     

Flag	Coverage Δ
backward-compatibility	0.08% <ø> (ø)
db-migration	0.08% <ø> (ø)
genesis-check	1.26% <ø> (ø)
integration-tests	36.18% <0.00%> (+0.05%)	⬆️
linux	71.63% <92.96%> (-0.03%)	⬇️
linux-nightly	71.36% <92.96%> (-0.01%)	⬇️
macos	55.00% <92.96%> (-0.03%)	⬇️
pytests	1.48% <ø> (ø)
sanity-checks	1.28% <ø> (ø)
unittests	68.14% <92.96%> (-0.03%)	⬇️
upgradability	0.13% <ø> (ø)

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