tests: remove various filters due to bugs/issue that were fixed

Some platforms were excluded due to issues that were fixed or resolved
themselves. Enable those platforms again and remove the comments related
to the issues.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>

tests/kernel/fatal/exception/testcase.yaml

@@ -43,8 +43,6 @@ tests:
       - mps2/an385
   kernel.common.stack_sentinel:
     extra_args: CONF_FILE=sentinel.conf
-    platform_exclude:
-      - qemu_cortex_a9 # FIXME: See issue #39948
     tags: kernel
     integration_platforms:
       - qemu_x86

tests/kernel/poll/testcase.yaml

@@ -4,10 +4,8 @@ tests:
     tags:
       - kernel
       - userspace
-    # FIXME: qemu_arc/qemu_arc_hs6x is excluded due to a run-time failure, see #49492
     platform_exclude:
       - nrf52dk/nrf52810
-      - qemu_arc/qemu_arc_hs6x
   kernel.poll.minimallibc:
     filter: CONFIG_MINIMAL_LIBC_SUPPORTED
     ignore_faults: true
@@ -15,9 +13,7 @@ tests:
       - kernel
       - userspace
       - libc
-    # FIXME: qemu_arc/qemu_arc_hs6x is excluded due to a run-time failure, see #49492
     platform_exclude:
       - nrf52dk/nrf52810
-      - qemu_arc/qemu_arc_hs6x
     extra_configs:
       - CONFIG_MINIMAL_LIBC=y

tests/kernel/tickless/tickless_concept/testcase.yaml

@@ -1,8 +1,6 @@
 tests:
   kernel.tickless.concept:
     arch_exclude: nios2
-    # FIXME: This test fails sporadically on all QEMU platforms, but fails
-    # consistently when coverage is enabled. Disable until 14173 is fixed.
     platform_exclude:
       - litex_vexriscv
       - rv32m1_vega/openisa_rv32m1/zero_riscy

tests/kernel/timer/timer_monotonic/testcase.yaml

@@ -3,9 +3,6 @@ tests:
     tags:
       - kernel
       - timer
-    # FIXME: This test may fail for qemu_arc/qemu_arc_hs on certain host systems.
-    #        See foss-for-synopsys-dwc-arc-processors/qemu#67.
-    platform_exclude: qemu_arc/qemu_arc_hs
   kernel.timer.monotonic.apic.tsc:
     tags:
       - kernel