test: fix RTC base time (#8214)

[mprse]

Description

Change the base time value to more realistic:
START_TIME = 1537789823 # GMT: Monday, 24 September 2018 11:50:23

This fix has been proposed by STM in order to enhance test efficiency.
Current test version did not detect problem with RTC reset on STM F1 family boards, since the base time was too small.

Pull request type

[X] Fix
[ ] Refactor
[ ] Target update
[ ] Functionality change
[ ] Breaking change

[bcostm]

I ran the rtc tests on the NUCLEO_F103RB platform and now the rtc_reset test is well FAIL as planned :)

+-------------------+---------------+------------------------------+--------+--------------------+-------------+
| target            | platform_name | test suite                   | result | elapsed_time (sec) | copy_method |
+-------------------+---------------+------------------------------+--------+--------------------+-------------+
| NUCLEO_F103RB-ARM | NUCLEO_F103RB | tests-mbed_drivers-rtc       | OK     | 30.73              | default     |
| NUCLEO_F103RB-ARM | NUCLEO_F103RB | tests-mbed_hal-rtc           | OK     | 50.62              | default     |
| NUCLEO_F103RB-ARM | NUCLEO_F103RB | tests-mbed_hal-rtc_reset     | FAIL   | 21.15              | default     |
| NUCLEO_F103RB-ARM | NUCLEO_F103RB | tests-mbed_hal-rtc_time      | OK     | 17.29              | default     |
| NUCLEO_F103RB-ARM | NUCLEO_F103RB | tests-mbed_hal-rtc_time_conv | OK     | 38.0               | default     |
+-------------------+---------------+------------------------------+--------+--------------------+-------------+

[NirSonnenschein]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 3260
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/8230/

Triggering tests

/morph test
/morph export-build
/morph mbed2-build

[mbed-ci]

Exporter Build : FAILURE

Build number : 2881
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/8230/

[mbed-ci]

Test : FAILURE

Build number : 3073
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/8230/3073

[NirSonnenschein]

/morph export-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 2895
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/8230/

[mprse]

only tests-mbed_hal-flash test has failed on EV_COG_AD3029LZ / ARM. This looks not related.

[adbridge]

/morph test

[mbed-ci]

Test : SUCCESS

Build number : 3144
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/8230/3144

[mbed-ci]

Exporter Build : FAILURE

Build number : 3071
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/8230/