Test spark lite heavy

[Chandanydv1234]

Closes #1275

What & why

Our Spark JUnits all shared one SparkSessionProvider that read
zingg.properties, which sets the driver/executor to 8g. Only the end-to-end
tests actually need that — the rest were paying for 8g for no reason. This
splits the provider into a base plus two memory profiles so unit tests can run
light.

Changes

• Added SparkSessionProviderBase (abstract) that holds all the session
  setup; subclasses just say which properties file to load via
  getPropertiesFile().
• SparkSessionProviderLite loads a new zingg-lite.properties (1g),
  SparkSessionProviderHeavy loads zingg-heavy.properties (8g, the old file
  renamed).
• Added TestSparkBaseLite / TestSparkBaseHeavy extensions.
• Moved the 18 unit tests onto TestSparkBaseLite, and the two integration
  executor tests (TestSparkExecutorsCompound, TestSparkExecutorsSingle)
  onto TestSparkBaseHeavy.
• Removed the old SparkSessionProvider and TestSparkBase.
• Set reuseForks=false in spark/core's surefire config. A Spark session is a
  singleton per JVM (getOrCreate hands back the first one built), so without a
  fresh JVM per test class the first class to run would fix the memory for the
  rest — this keeps the 1g and 8g profiles independent.

Tests

Ran locally with mvn -pl spark/core -am test. The provider logs
Spark driver memory: X GB on startup, so the numbers below are what each
session actually came up with:

• test-compile for spark/core + upstream modules — compiles clean.
• TestSparkFrame (Lite) — 1 GB — 32 run, 0 failures, 2 skipped.
• TestSparkExecutorsCompound (Heavy) — 8 GB — 1 run, 0 failures.
• TestSparkExecutorsSingle (Heavy) — 8 GB — 1 run, 0 failures.
• Sanity-checked the original code behaves the same, so no regression.

One open question

The issue says "only the integration test" for Heavy, but I put both
TestSparkExecutorsCompound and TestSparkExecutorsSingle on Heavy since both
run full pipelines off configSparkIntTest.json. Happy to move Single back to
Lite if you'd rather stick to the letter of the issue.

[sonalgoyal]

instead of a static getInstance method, can you have a static attribute named isSessionInitialised just like https://www.baeldung.com/java-junit5-run-before-all-classes ?

[Chandanydv1234]

instead of a static getInstance method, can you have a static attribute named isSessionInitialised just like https://www.baeldung.com/java-junit5-run-before-all-classes ?

Done as suggested