all: make tests pass on the debian-minimal docker image

Such a minimal system has no extra locales installed,
so trying to use "en_US.UTF-8" might fail.
Ideally we would want the standard "C" locale,
as it is meant for consumption by computers,
but unfortunately it also means no built-in UTF-8.

However, there's good news; "C.UTF-8" is emerging for our use case.
Many systems, like Debian, already ship with it by default.
glibc won't include it until 2.35, and Arch ships 2.33,
so for the time being, Arch Linux and some other distros lack it.
For their sake and mine, fall back to our previous locale.

Note that this backport excludes the interp/os_unix.go change in v3.5.
It seems a bit too risky for a bugfix backport.

Fixes #773.

interp/interp_test.go

@@ -130,8 +130,18 @@ func TestMain(m *testing.M) {
 	}
 	os.Setenv("GOSH_PROG", prog)
 
-	os.Setenv("LANGUAGE", "en_US.UTF-8")
-	os.Setenv("LC_ALL", "en_US.UTF-8")
+	// Set the locale to computer-friendly English and UTF-8.
+	// Some systems like Arch miss C.UTF8, so fall back to the US English locale.
+	if out, _ := exec.Command("locale", "-a").Output(); strings.Contains(
+		strings.ToLower(string(out)), "c.utf",
+	) {
+		os.Setenv("LANGUAGE", "C.UTF-8")
+		os.Setenv("LC_ALL", "C.UTF-8")
+	} else {
+		os.Setenv("LANGUAGE", "en_US.UTF-8")
+		os.Setenv("LC_ALL", "en_US.UTF-8")
+	}
+
 	os.Unsetenv("CDPATH")
 	hasBash50 = checkBash()
 
@@ -2775,7 +2785,7 @@ set +o pipefail
 }
 
 var runTestsUnix = []runTest{
-	{"[[ -n $PPID && $PPID -gt 0 ]]", ""},
+	{"[[ -n $PPID && $PPID -ge 0 ]]", ""}, // can be 0 if running as the init process
 	{
 		// no root user on windows
 		"[[ ~root == '~root' ]]",

syntax/parser_test.go

@@ -155,8 +155,17 @@ func TestParseBats(t *testing.T) {
 }
 
 func TestMain(m *testing.M) {
-	os.Setenv("LANGUAGE", "en_US.UTF8")
-	os.Setenv("LC_ALL", "en_US.UTF8")
+	// Set the locale to computer-friendly English and UTF-8.
+	// Some systems like Arch miss C.UTF8, so fall back to the US English locale.
+	if out, _ := exec.Command("locale", "-a").Output(); strings.Contains(
+		strings.ToLower(string(out)), "c.utf",
+	) {
+		os.Setenv("LANGUAGE", "C.UTF-8")
+		os.Setenv("LC_ALL", "C.UTF-8")
+	} else {
+		os.Setenv("LANGUAGE", "en_US.UTF-8")
+		os.Setenv("LC_ALL", "en_US.UTF-8")
+	}
 	os.Exit(m.Run())
 }