testsuite/systemtap.base/perf.sh drop bashism

testsuite/systemtap.base/perf.sh drop bashism

You don't need to use == to check for equality in a test statement, a
single equals sign is sufficient. The use of double equals sign is a
bashism which doesn't always translate as intended on other shells.

[fche checked other scripts in the source tree that used [ == ]; they
are all marked /bin/bash so can stay as is.]