Fix build hang on .validation.test

[chanseokoh]

Fixes #1527.

About org.eclispe.platform: I had originally thought using org.eclipse.platform instead of org.eclipse.rcp somehow eliminated the build hang (#1484 (comment)), but it wasn't the case, so I am reverting that.

[chanseokoh]

FTR, one build failure above is due to a different reason.

[briandealwis]

Nice!

[briandealwis]

This wasn't required previously?

[chanseokoh]

The test was working fine previously (which didn't have the marker checking). The facets are for actually triggering validation in the IDE when a file is open in an editor. It looks like the project needs to be an App Engine project to trigger it (and the file name is appengine-web.xml).

[codecov-io]

Codecov Report

Merging #1553 into master will increase coverage by 0.51%.
The diff coverage is 50%.

@@             Coverage Diff              @@
##             master    #1553      +/-   ##
============================================
+ Coverage     70.53%   71.05%   +0.51%     
- Complexity     1307     1328      +21     
============================================
  Files           233      233              
  Lines          8995     9139     +144     
  Branches        765      785      +20     
============================================
+ Hits           6345     6494     +149     
- Misses         2326     2327       +1     
+ Partials        324      318       -6

Impacted Files	Coverage Δ	Complexity Δ
...gle/cloud/tools/eclipse/ui/util/WorkbenchUtil.java	64% <50%> (-8%)	4 <0> (ø)
.../tools/eclipse/test/util/project/ProjectUtils.java	80.61% <0%> (+1.02%)	19% <0%> (+1%)	✅
...ipse/appengine/libraries/model/LibraryFactory.java	72.83% <0%> (+1.23%)	17% <0%> (+1%)	✅
...gine/deploy/ui/StandardDeployPreferencesPanel.java	87.84% <0%> (+2.03%)	33% <0%> (+10%)	✅
...m/google/cloud/tools/eclipse/ui/util/FontUtil.java	79.16% <0%> (+2.69%)	6% <0%> (+1%)	✅
...tools/eclipse/projectselector/ProjectSelector.java	92.42% <0%> (+2.76%)	9% <0%> (ø)	❌
...e/cloud/tools/eclipse/appengine/libraries/Pom.java	86.74% <0%> (+4.65%)	15% <0%> (+2%)	✅
...m/google/cloud/tools/eclipse/util/NatureUtils.java	88.88% <0%> (+11.11%)	5% <0%> (+1%)	✅
...appengine/facets/NonSystemJobScheduleListener.java	100% <0%> (+12.5%)	7% <0%> (+1%)	✅
...pengine/validation/AbstractXmlSourceValidator.java	92% <0%> (+40%)	8% <0%> (+4%)	✅

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