Gh-3108: Improve testing for GafferPopVertexProperty

[tb06904]

Improves the testing on GafferPopVertexProperty class to above > 80% coverage.

Slight tweak to reduce mock duplication in the existing test class too.

Related issue

• Resolve Improve testing for GafferPopVertexProperty #3108

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (a0dcee3) 65.74% compared to head (b56f27f) 65.79%.

Additional details and impacted files

@@              Coverage Diff              @@
##             develop    #3110      +/-   ##
=============================================
+ Coverage      65.74%   65.79%   +0.05%     
- Complexity      2512     2514       +2     
=============================================
  Files            909      909              
  Lines          29008    29004       -4     
  Branches        3237     3235       -2     
=============================================
+ Hits           19070    19083      +13     
+ Misses          8488     8475      -13     
+ Partials        1450     1446       -4     

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[t92549]

There is now only this that remains with no coverage:

Gaffer/library/tinkerpop/src/main/java/uk/gov/gchq/gaffer/tinkerpop/GafferPopVertexProperty.java

Lines 113 to 116 in 1ef3621

	if (propertyKeys.length == 1) {
	final Property<U> property = properties.get(propertyKeys[0]);
	return null == property ? Collections.emptyIterator() : IteratorUtils.of(property);
	}

If I am honest I don't really understand this. It might be good to either test it or remove it?