Add more Sources tests

[thuydotm]

Changes addressed in this PR:

• split catalog to different files to test IntakeSource and IntakeSQLSource separately
• move tests for FileSource to a new file
• add tests to JSONSource
• add tests to JoinedSource
• move common testing logics to a utils file
• test Source.from_spec when loading from source in state.sources

Bug fixes:

• JSONSource._resolve_template_vars(): convert template to string to make sure re.findall(template) work properly. Currently, a local file path will be read as a pathlib.PosixPath object: https://github.com/holoviz/lumen/blob/master/lumen/sources/base.py#L528

[codecov-commenter]

Codecov Report

Merging #309 (8c50ba6) into master (f94ee65) will increase coverage by 0.53%.
The diff coverage is 98.42%.

@@            Coverage Diff             @@
##           master     #309      +/-   ##
==========================================
+ Coverage   66.26%   66.80%   +0.53%     
==========================================
  Files          59       62       +3     
  Lines        6178     6250      +72     
==========================================
+ Hits         4094     4175      +81     
+ Misses       2084     2075       -9     

Impacted Files	Coverage Δ
lumen/tests/sources/test_intake.py	92.68% <86.95%> (-7.32%)	⬇️
lumen/tests/conftest.py	97.54% <92.85%> (-0.63%)	⬇️
lumen/sources/base.py	66.00% <100.00%> (+3.34%)	⬆️
lumen/tests/sources/test_base.py	100.00% <100.00%> (+0.94%)	⬆️
lumen/tests/sources/test_derived.py	100.00% <100.00%> (ø)
lumen/tests/sources/test_file.py	100.00% <100.00%> (ø)
lumen/tests/sources/test_intake_sql.py	100.00% <100.00%> (ø)
lumen/tests/sources/test_json.py	100.00% <100.00%> (ø)
lumen/tests/sources/utils.py	100.00% <100.00%> (ø)
lumen/sources/intake_sql.py	83.78% <0.00%> (-5.41%)	⬇️
... and 3 more

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more