perf test: Replace legacy `...` with $(...)

As detailed in https://www.shellcheck.net/wiki/SC2006:

The use of `...` is legacy syntax with several issues:
1. It has a series of undefined behaviors related to quoting in POSIX.
2. It imposes a custom escaping mode with surprising results.
3. It's exceptionally hard to nest.

$(...) command substitution has none of these problems,
and is therefore strongly encouraged.
Signed-off-by: Diederik de Haas <didi.debian@cknow.org>
Acked-by: Carsten Haitzler <carsten.haitzler@arm.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20230201214945.127474-3-didi.debian@cknow.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/tests/shell/lib/coresight.sh

@@ -58,9 +58,9 @@ perf_dump_aux_verify() {
 	# compiler may produce different code depending on the compiler and
 	# optimization options, so this is rough just to see if we're
 	# either missing almost all the data or all of it
-	ATOM_FX_NUM=`grep -c I_ATOM_F "$DUMP"`
-	ASYNC_NUM=`grep -c I_ASYNC "$DUMP"`
-	TRACE_INFO_NUM=`grep -c I_TRACE_INFO "$DUMP"`
+	ATOM_FX_NUM=$(grep -c I_ATOM_F "$DUMP")
+	ASYNC_NUM=$(grep -c I_ASYNC "$DUMP")
+	TRACE_INFO_NUM=$(grep -c I_TRACE_INFO "$DUMP")
 	rm -f "$DUMP"
 
 	# Arguments provide minimums for a pass
@@ -96,18 +96,18 @@ perf_dump_aux_tid_verify() {
 
 	# The TID test tools will print a TID per stdout line that are being
 	# tested
-	TIDS=`cat "$2"`
+	TIDS=$(cat "$2")
 	# Scan the perf report to find the TIDs that are actually CID in hex
 	# and build a list of the ones found
-	FOUND_TIDS=`perf report --stdio --dump -i "$1" | \
+	FOUND_TIDS=$(perf report --stdio --dump -i "$1" | \
 			grep -o "CID=0x[0-9a-z]\+" | sed 's/CID=//g' | \
-			uniq | sort | uniq`
+			uniq | sort | uniq)
 	# No CID=xxx found - maybe your kernel is reporting these as
 	# VMID=xxx so look there
 	if test -z "$FOUND_TIDS"; then
-		FOUND_TIDS=`perf report --stdio --dump -i "$1" | \
+		FOUND_TIDS=$(perf report --stdio --dump -i "$1" | \
 				grep -o "VMID=0x[0-9a-z]\+" | sed 's/VMID=//g' | \
-				uniq | sort | uniq`
+				uniq | sort | uniq)
 	fi
 
 	# Iterate over the list of TIDs that the test says it has and find
@@ -116,7 +116,7 @@ perf_dump_aux_tid_verify() {
 	for TID2 in $TIDS; do
 		FOUND=""
 		for TIDHEX in $FOUND_TIDS; do
-			TID=`printf "%i" $TIDHEX`
+			TID=$(printf "%i" $TIDHEX)
 			if test "$TID" -eq "$TID2"; then
 				FOUND="y"
 				break