SI-5938 Test for a FSC bug with mixin, duplicate static forwarders

Under resident compilation, we were getting multiple copies of static
forwarders created for mixed in methods. It seems like the bug was
fixed as a by-product of #4040.

This commit adds a test to show this. I've confirmed that the test
fails appropriately with 2.11.4.

For future reference, before I figured out how to write the test
for this one (test/resident doesn't seem to let you run the code
after compiling it), I was using bash to test as follows:

    (export V=2.11.x; (scalac-hash $V sandbox/t5938_1.scala; (for i in 1 2; do echo sandbox/t5938.scala; done; printf '\n') | scalac-hash $V  -Xresident); stty echo; scala-hash $V X ; echo ':javap -public X' | scala-hash $V);

Review by @gkossakowski