RFR: Fix a crash with IntelliJ 2020.2

[Liam Miller-Cushon]

com.intellij.diagnostic.PluginException: Class constructor must not have parameters: com.oracle.plugin.jtreg.service.ui.JTRegServiceConfigurable [Plugin: jtreg]
	at com.intellij.serviceContainer.ComponentManagerImpl.instantiateExtensionWithPicoContainerOnlyIfNeeded(ComponentManagerImpl.kt:696)
	at com.intellij.openapi.options.ConfigurableEP$ClassProducer.createElement(ConfigurableEP.java:422)
	at com.intellij.openapi.options.ConfigurableEP.createConfigurable(ConfigurableEP.java:329)
	at com.intellij.openapi.options.ex.ConfigurableWrapper.createConfigurable(ConfigurableWrapper.java:45)

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Commit messages:
 - Fix a crash with IntelliJ 2020.2

Changes: https://git.openjdk.java.net/jtreg/pull/5/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jtreg&pr=5&range=00
  Stats: 19 lines in 1 file changed: 13 ins; 3 del; 3 mod
  Patch: https://git.openjdk.java.net/jtreg/pull/5.diff
  Fetch: git fetch https://git.openjdk.java.net/jtreg pull/5/head:pull/5

PR: https://git.openjdk.java.net/jtreg/pull/5