config: rt-tests: fix MLOCKALL getting set to True

When MLOCKALL is set to 'true', the lava job has MLOCKALL becomes true
[1] which in turn translates to "True" while job execution [2]. It
causes error as the test expects true instead of True.

  ./pi-stress.sh -D 60s -m True -r false -w hackbench
  ./pi-stress.sh: 37: True: not found

Fix it by using double quotes inside single quotes.
- First set of quotations allows Jinja template engine to treat it as a
  string
- Second set is for LAVA to also treat is as string and prevent YAML
  parser from making it a bool

[3] is successful job after this change.

[1] https://lava.collabora.dev/scheduler/job/15562666/definition#defline43
[2] https://lava.collabora.dev/scheduler/job/15562666#L220
[3] https://lava.collabora.dev/scheduler/job/15790058

Relates kernelci/kernelci-project#439
Co-authored-by: Paweł Wieczorek <[email protected]>
Signed-off-by: Muhammad Usama Anjum <[email protected]>

config/runtime/tests/rt-tests.jinja2

@@ -25,7 +25,7 @@
         PRIORITY: {{ priority|default('98') }}
         THREADS: {{ threads|default('2') }}
       {% elif tst_cmd == 'pi-stress' %}
-        MLOCKALL: {{ mlockall|default('true') }}
+        MLOCKALL: {{ mlockall|default('"true"') }}
       {% elif tst_cmd == 'rt-migrate-test' %}
         PRIORITY: {{ priority|default('90') }}
       {% elif tst_cmd == 'signaltest' %}