Tests | Junit improvements + Organize constants

[cheenamalhotra]

This PR includes changes as under:

• Manage Constants in one common class and use static reference to access them across all tests.
• Removed magic strings and numerics in tests to use static constants instead.
• Created getConnection() method in AbstractTest class that returns Connection using available connectionString and updated tests to use this instead of creating connection from DriverManager.
• Updated other custom connection requests to get connection from PrepUtil class.
• Updated e.toString() to e.getMessage()
• Closed open statement(s) in some files.
• Removed unused methods (we are using alternative methods to do the same stuff)
• Organize imports + fixed warnings + cleanups.

[codecov-io]

Codecov Report

Merging #982 into dev will decrease coverage by 0.05%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                dev     #982      +/-   ##
============================================
- Coverage     50.14%   50.08%   -0.06%     
+ Complexity     2877     2872       -5     
============================================
  Files           120      120              
  Lines         27981    27981              
  Branches       4677     4677              
============================================
- Hits          14030    14015      -15     
- Misses        11690    11712      +22     
+ Partials       2261     2254       -7

Flag	Coverage Δ	Complexity Δ
#JDBC42	49.69% <100%> (+0.07%)	2835 <1> (+2)	⬆️
#JDBC43	50.02% <100%> (-0.03%)	2869 <1> (-2)

Impacted Files	Coverage Δ	Complexity Δ
...m/microsoft/sqlserver/jdbc/SQLServerStatement.java	60.9% <ø> (ø)	137 <0> (ø)	⬇️
...om/microsoft/sqlserver/jdbc/SQLServerMetaData.java	21.81% <ø> (ø)	0 <0> (ø)	⬇️
...oft/sqlserver/jdbc/SQLServerResultSetMetaData.java	45.52% <ø> (ø)	27 <0> (ø)	⬇️
...m/microsoft/sqlserver/jdbc/SQLServerSavepoint.java	90% <ø> (ø)	10 <0> (ø)	⬇️
...n/java/com/microsoft/sqlserver/jdbc/tdsparser.java	68.1% <100%> (ø)	0 <0> (ø)	⬇️
...ava/com/microsoft/sqlserver/jdbc/SQLServerLob.java	100% <100%> (ø)	1 <1> (ø)	⬇️
...java/com/microsoft/sqlserver/jdbc/StringUtils.java	0% <0%> (-33.34%)	2% <0%> (-1%)
...rc/main/java/com/microsoft/sqlserver/jdbc/DDC.java	46.22% <0%> (-1.73%)	110% <0%> (-2%)
...m/microsoft/sqlserver/jdbc/SQLServerException.java	76.64% <0%> (-1.46%)	32% <0%> (-1%)
...m/microsoft/sqlserver/jdbc/SQLServerResultSet.java	43.86% <0%> (-0.2%)	318% <0%> (-2%)
... and 6 more

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