perf expr: Initialize is_test value in expr__ctx_new()

perf expr: Initialize is_test value in expr__ctx_new()

When expr_parse_ctx is allocated by expr_ctx_new(),
expr_scanner_ctx->is_test isn't initialize, so it has garbage value.
this can affects the result of expr__parse() return when it parses
non-exist event literal according to garbage value.

Use calloc instead of malloc in expr_ctx_new() to fix this.

Fixes: 3340a08354ac286e ("perf pmu-events: Fix testing with JEVENTS_ARCH=all")
Reviewed-by: Ian Rogers <irogers@google.com>
Reviewed-by: James Clark <james.clark@linaro.org>
Signed-off-by: Levi Yun <yeoreum.yun@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20241108143424.819126-1-yeoreum.yun@arm.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>