Add an env variable to control running tests from travis ui
Add an env variable to control running tests from travis ui

While working on https://github.com/pantsbuild/intellij-pants-plugin/issues/130,
there were tests mysteriously failing, running the entire test suite takes 20minutes
and is not reproducible locally, this allows us to better reproduce problems on travis.

Testing Done:
Tried in CI, an example usage is to set `ADDITIONAL_ARGS` to a small set of tests, like

`"--test-junit-test=com.twitter.intellij.pants.completion.PantsCompletionTest --test-junit-test=com.twitter.intellij.pants.components.impl.PantsProjectCacheTest --test-junit-test=com.twitter.intellij.pants.execution.OSSPantsExamplesRunConfigurationIntegrationTest --test-junit-test=com.twitter.intellij.pants.execution.PantsOutputMessageTest --test-junit-test=com.twitter.intellij.pants.highlighting.PantsHighlightingIntegrationTest --test-junit-test=com.twitter.intellij.pants.highlighting.PantsUnresolvedJavaReferenceQuickFixProviderTest --test-junit-test=com.twitter.intellij.pants.highlighting.PantsUnresolvedScalaReferenceQuickFixProviderTest"`

Bugs closed: 131

Reviewed at https://rbcommons.com/s/twitter/r/3824/

pantsbuild/intellij-pants-plugin,wisechengyi/intellij-pants-plugin,pantsbuild/intellij-pants-plugin,wisechengyi/intellij-pants-plugin,pantsbuild/intellij-pants-plugin,wisechengyi/intellij-pants-plugin,pantsbuild/intellij-pants-plugin,pantsbuild/intellij-pants-plugin,wisechengyi/intellij-pants-plugin