tools: tc-testing: Can refer to $TESTID in test spec

When processing the commands in the test cases, substitute
the test id for $TESTID.  This helps to make more flexible
tests.  For example, the testid can be given as a command
line argument.

As an example, if we wish to save the test output to a file
named for the test case, we can write in the test case:

"cmdUnderTest": "some test command | tee -a $TESTID.out"

Signed-off-by: Brenda J. Butler <bjb@mojatatu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>