[test] Log to ETW for benchmarks; retry layer removal (microsoft#1947)

Write logs to ETW instead of stdout when running benchmarks to mimic
actual deployments.
Add log level to `Run-Test.ps1` helper script.

Add `RemoveAll` function that wraps and retries `os.RemoveAll` (waiting
in between attempts) since the OS may take a while to remove locks on a
directory after handles are closed.

Bug: defer uVM cleanup after creation (not start) during tests.

Signed-off-by: Hamza El-Saawy <hamzaelsaawy@microsoft.com>

log/nopformatter.go

@@ -0,0 +1,12 @@
+package log
+
+import (
+ "github.com/sirupsen/logrus"
+)
+
+type NopFormatter struct{}
+
+var _ logrus.Formatter = NopFormatter{}
+
+// Format does nothing and returns a nil slice.
+func (NopFormatter) Format(*logrus.Entry) ([]byte, error) { return nil, nil }