t9350: skip ISO-8859-1 test when the environment is always-UTF-8

In the BusyBox-w32 version that is currently under consideration for
MinGit for Windows (to reduce the .zip size, and to avoid problems with
the MSYS2 runtime), the UTF-16 environment present in Windows is
considered to be authoritative, and the 8-bit version is always in UTF-8
encoding.

As a consequence, the ISO-8859-1 test in t9350-fast-export (which tries
to set GIT_AUTHOR_NAME to a ISO-8859-1 encoded value) *must* fail in
that setup.

So let's detect when it would fail (due to an environment being purely
kept UTF-8 encoded), and skip that test in that case.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

t/t9350-fast-export.sh

@@ -66,7 +66,12 @@ test_expect_success 'fast-export master~2..master' '
 
 '
 
-test_expect_success 'iso-8859-1' '
+test_lazy_prereq UTF8_ONLY_ENV '
+	. "$TEST_DIRECTORY"/t3901/8859-1.txt &&
+	! git var GIT_AUTHOR_IDENT | grep "Áéí"
+'
+
+test_expect_success !UTF8_ONLY_ENV 'iso-8859-1' '
 
 	git config i18n.commitencoding ISO8859-1 &&
 	# use author and committer name in ISO-8859-1 to match it.
@@ -82,6 +87,11 @@ test_expect_success 'iso-8859-1' '
 		 grep "Áéí óú" actual)
 
 '
+
+# The subsequent tests validate timestamps, and we may just have skipped a tick
+test_have_prereq !UTF8_ONLY_ENV ||
+test_tick
+
 test_expect_success 'import/export-marks' '
 
 	git checkout -b marks master &&
@@ -196,7 +206,7 @@ GIT_COMMITTER_NAME='C O Mitter'; export GIT_COMMITTER_NAME
 
 test_expect_success 'setup copies' '
 
-	git config --unset i18n.commitencoding &&
+	{ git config --unset i18n.commitencoding || :; } &&
 	git checkout -b copy rein &&
 	git mv file file3 &&
 	git commit -m move1 &&