support/testing: TestNoTimezone: fix the test case for Glibc

We have changed to a Glibc based toolchain recently [1] but the
behavior of TZ handling is not the same between libc implementation
when no Zone Database is installed.

musl and uClibc-ng return "UTC" when the data file of the requested
time zone is missing or when TZ is not set.

 # TZ=America/Los_Angeles date +%Z
 UTC

 # TZ= date +%Z
 UTC

Glibc return all or part of TZ content or "Universal" if TZ is empty.

 # TZ=America/Los_Angeles date +%Z
 America

 # TZ= date +%Z
 Universal

As demonstrated by TestAllTimezone, Glibc return "PDT" when the
America/Los_Angeles time zone data file is installed:

 # TZ=America/Los_Angeles date +%Z
 PDT

Since the Glibc behavior seems weird (not a bug [2]) when TZ is
set but the time zone data file is missing, update our test
to check against a string defined in the Glibc code [3].

[1] f89f52168fcb667a3e8e43f6f44d5b1ca3961a8c
[2] https://sourceware.org/bugzilla/show_bug.cgi?id=30710
[3] https://sourceware.org/git/?p=glibc.git;a=blob;f=time/tzset.c;h=78c18f8147415c92dc6eb735be672fa7e0b8f76e;hb=47b76f6d1d3a5ad13e585dbcc616aaea62b8bb20#l380

Ref:
https://www.gnu.org/software/libc/manual/html_node/TZ-Variable.html

Fixe:
https://gitlab.com/buildroot.org/buildroot/-/jobs/4768561117

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

support/testing/tests/core/test_timezone.py

@@ -22,8 +22,11 @@ class TestNoTimezone(infra.basetest.BRTest):
         boot_armv5_cpio(self.emulator, self.builddir)
         tz, _ = self.emulator.run("TZ=UTC date +%Z")
         self.assertEqual(tz[0].strip(), "UTC")
-        tz, _ = self.emulator.run("TZ=America/Los_Angeles date +%Z")
-        self.assertEqual(tz[0].strip(), "UTC")
+        # This test is Glibc specific since there is no Time Zone Database installed
+        # and other C libraries use their own rule for returning time zone name or
+        # abbreviation when TZ is empty or set with a not installed time zone data file.
+        tz, _ = self.emulator.run("TZ= date +%Z")
+        self.assertEqual(tz[0].strip(), "Universal")
 
 
 class TestAllTimezone(infra.basetest.BRTest):