Allow forcing colored system test output

Allow forcing colored system test output

As parallel.mk and runsequential.sh both pipe system test output through
"tee" (for the purpose of creating test.output), run.sh invoked from
these two files detects it is not writing to a terminal, which prevents
colored output from being generated.  Allow forcing colored output using
a new command line argument for runall.sh, "-c", which sets an
environment variable (SYSTEMTEST_FORCE_COLOR) causing conf.sh to
unconditionally enable colored output.

The same environment variable can also be used directly to force colored
output when using "make test" instead of runall.sh.

(cherry picked from commit a324031a82a2b312d8f803040dd81eadb554785f)