ta: os_test: do not print caller function name in EMSG()

EMSG() already prints the caller function name (and the line
number). As such, no need to hardcode it.

Remove all usage of caller function name in EMSG().

Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Reviewed-by: Etienne Carriere <etienne.carriere@foss.st.com>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>