test: Use prereq test for git send-email

Some platforms (e.g. Ubuntu) do not package `git send-email` with the
rest of git. And some CI platforms (e.g. GitHub Actions) use a version
of git that is incompatible with the distro-provided git-email package.

So since we cannot count on the availability of `git send-email` in all
environments, add GITSENDEMAIL prereq checks to relevant tests that need
`git send-email`.

Author: jpgrayson

t/t1903-email-send.sh

@@ -17,7 +17,7 @@ Subject: [PATCH 2/4] p2
 Subject: [PATCH 3/4] p3
 Subject: [PATCH 4/4] p4
 EOF
-test_expect_success 'Send all applied patches' '
+test_expect_success GITSENDEMAIL 'Send all applied patches' '
     stg email send --dry-run --to [email protected] --all >out &&
     grep "Subject: " out > subjects &&
     test_cmp expected subjects
@@ -28,7 +28,7 @@ Subject: [PATCH 1/3] p3
 Subject: [PATCH 2/3] p4
 Subject: [PATCH 3/3] p5
 EOF
-test_expect_success 'Send custom patch range' '
+test_expect_success GITSENDEMAIL 'Send custom patch range' '
     stg email send --dry-run --to [email protected] p3..p5 >out &&
     grep "Subject: " out > subjects &&
     test_cmp expected subjects
@@ -37,7 +37,7 @@ test_expect_success 'Send custom patch range' '
 cat >expected <<EOF
 Subject: [PATCH] p7
 EOF
-test_expect_success 'Send single patch' '
+test_expect_success GITSENDEMAIL 'Send single patch' '
     stg email send --dry-run --to [email protected] p7 >out &&
     grep "Subject: " out > subjects &&
     test_cmp expected subjects
@@ -56,7 +56,7 @@ Subject: [PATCH 1/3] other1
 Subject: [PATCH 2/3] other2
 Subject: [PATCH 3/3] other3
 EOF
-test_expect_success 'Send patches from other branch' '
+test_expect_success GITSENDEMAIL 'Send patches from other branch' '
     stg email send --dry-run --to [email protected] --branch other --all >out &&
     grep "Subject: " out > subjects &&
     test_cmp expected subjects

t/test-lib.sh

@@ -1329,3 +1329,8 @@ test_lazy_prereq QUILT '
 	# test whether quilt is installed
 	quilt --version 2>/dev/null >/dev/null
 '
+
+test_lazy_prereq GITSENDEMAIL '
+	# test whether git send-email is available
+	git send-email --help 2>/dev/null >/dev/null
+'