[MemProf] Augment test to debug avr bot failure

After 81f7b96e, I can see that the
reason this test is failing on llvm-avr-linux is that it doesn't think
the directory exists (error comes during file open for write command).
Not sure why since this is the main test Output directory and we created
a different file there earlier in the test from the same file open
invocation. Print directory contents in an attempt to debug.