[testing] Fix rstmgr alert_info test
[testing] Fix rstmgr alert_info test

This commit fixes the following issues in the rstmgr alert_info test:
* Control flow bugs in test_main:
  * Returns `true` if an unexpected `rst_info` is encountered.
    Generally, it's safer to fail by default in a multi-step test like
    this and return `true` from a single branch if possible.
  * Returns `true` if the chip fails to reset by the expected delay.
* Print bug for `alert_cause`: Size of the `alert_cause` is 64 bits while
the `%x` format specifier is clearly documented as intended for
`uint32_t`s.
* `merge32()` was wrong and not really needed.
* `rst_info` check should be relaxed for the current bootstrap
implementation.
* Various style and formatting issues.

Signed-off-by: Alphan Ulusoy <23b245cc5a07aacf75a9db847b24c67dee1707bf@google.com>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan