perf tests: Use shebangs in the shell scripts

Since the first line was used as a test identification, it needs to be
skipped by shell_test__description() function now.

Further notes from Hendrik:

It might be worth to note that adding the shebang is necessary to spot
them as scripts.

Using /bin/sh looks fine to.  Just briefly checked whether the scripts
contains some bash-specifics, which is not the case.

Signed-off-by: Michael Petlan <mpetlan@redhat.com>
Reviewed-by: Hendrik Brueckner <brueckner@linux.ibm.com>
Cc: Jiri Olsa <jolsa@redhat.com>
LPU-Reference: 2127419430.57657104.1542836358464.JavaMail.zimbra@redhat.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>