[SPARK-22904][SQL] Add tests for decimal operations and string casts

[mgaido91]

What changes were proposed in this pull request?

Test coverage for arithmetic operations leading to:

1. Precision loss
2. Overflow

Moreover, tests for casting bad string to other input types and for using bad string as operators of some functions.

How was this patch tested?

added tests

[SparkQA]

Test build #85412 has finished for PR 20084 at commit 7a9650a.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #85433 has finished for PR 20084 at commit 103c0e7.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[mgaido91]

cc @gatorsmile

[gatorsmile]

LGTM

[gatorsmile]

Thanks! Merged to master.