tests: add PID of VPP under test to API dump filename for the test

tests: add PID of VPP under test to API dump filename for the test

Some testclasses (e.g. IPSec) have methods with the same names,
this causes a race when running in parallel to save the API and
move it into the test's directory - the name of the file has
only the test method name as a discriminator.

Saving two traces from two VPP instances "succeeds", in that
it silently overwrites the file. But only one mv operation
works - the other gives an error "file not found" and fails the test.

Solution: add the PID of VPP under test to the API dump filename,
this way the filenames become guaranteed unique and the race is avoided.

Change-Id: I1bb47ed73369fea0f1bfbf907588a51e2cabfb7f
Type: test
Signed-off-by: Andrew Yourtchenko <ayourtch@gmail.com>