Try to fix issue with MainTest.file is modified if it is not formatted (#309)

Summary:
Pull Request resolved: #309

GitHub actions are failing on this test case:
https://github.com/facebookincubator/ktfmt/runs/6005912425?check_suite_focus=true#step:4:93991

Looks like recent optimizations, and a different order of running stuff can make this test run too fast? I'm not sure, but I'm going to give this a try. If not, I'll run some other debugging ideas for this test.

Reviewed By: cgrushko

Differential Revision: D35611158

fbshipit-source-id: 816cecf57ba0b8451bdfe6d5f4122b707cc17c57

Author: strulovich

core/src/test/java/com/facebook/ktfmt/cli/MainTest.kt

@@ -188,6 +188,8 @@ class MainTest {
 
     val lastModifiedTimeBeforeRunningFormatter =
         Files.getLastModifiedTime(unformattedFilePath).toMillis()
+    // The test may run under 1ms, and we need to make sure the new file timestamp will be different
+    Thread.sleep(100)
     Main(emptyInput, PrintStream(out), PrintStream(err), arrayOf(unformattedFile.toString())).run()
     val lastModifiedTimeAfterRunningFormatter =
         Files.getLastModifiedTime(unformattedFilePath).toMillis()