tests+samples: Check for --help support to determine stat/sed tools flavor

Using the kernel name was not a good choice for determining which flavor
(BSD vs. GNU) of stat, sed, and install was being used since one could
install either one of them at least on MacOS. Instead, check whether
--help is supported by these tools.

Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>

samples/swtpm-create-tpmca

@@ -26,7 +26,7 @@ logerr()
 # @1: filename
 function get_filesize()
 {
-	if [[ "$(uname -s)" =~ (Linux|CYGWIN_NT-) ]]; then
+	if stat --help &>/dev/null; then
 		stat -c%s "$1"
 	else
 		# OpenBSD

tests/common

@@ -735,7 +735,7 @@ function run_swtpm_bios()
 # @1: filename
 function get_filesize()
 {
-	if [[ "$(uname -s)" =~ (Linux|CYGWIN_NT-|GNU) ]]; then
+	if stat --help &>/dev/null; then
 		stat -c%s "$1"
 	else
 		# OpenBSD
@@ -748,7 +748,7 @@ function get_filesize()
 # @1: filename
 function get_filemode()
 {
-	if [[ "$(uname -s)" =~ (Linux|CYGWIN_NT-) ]]; then
+	if stat --help &>/dev/null; then
 		stat -c%a "$1"
 	else
 		# BSDs
@@ -761,7 +761,7 @@ function get_filemode()
 # @1: filename
 function get_fileowner()
 {
-	if [[ "$(uname -s)" =~ (Linux|CYGWIN_NT-|GNU) ]]; then
+	if stat --help &>/dev/null; then
 		stat -c"%u %g" "$1"
 	else
 		# BSDs
@@ -774,7 +774,7 @@ function get_fileowner()
 # @1: filename
 function get_fileowner_names()
 {
-	if [[ "$(uname -s)" =~ (Linux|CYGWIN_NT-|GNU) ]]; then
+	if stat --help &>/dev/null; then
 		stat -c"%U %G" "$1"
 	else
 		# BSDs

tests/fileinstall

@@ -1,6 +1,6 @@
 #!/usr/bin/env bash
 
-if [[ "$(uname -s)" =~ (Linux|CYGWIN_NT-) ]]; then
+if install --help &>/dev/null; then
 	install -D "$1" "$2"
 else
 	mkdir -p "$(dirname "$2")"

tests/sed-inplace

@@ -1,6 +1,6 @@
 #!/usr/bin/env bash
 
-if [[ "$(uname -s)" =~ (Linux|CYGWIN_NT-|GNU) ]]; then
+if sed --help &>/dev/null; then
 	sed -i "$1" "$2"
 else
 	sed -i '' "$1" "$2"