Fix for MDEV-25820:

Tests will be skipped unless the following environment variables
were specified:

- MYSQLTEST_VARDIR (will be automatically set when running inside
  mtr)
- MARIADB_CC_TEST

appveyor.yml

@@ -24,6 +24,7 @@ before_build:
   - cmd: appveyor-download.bat
   - cmd: msiexec /i server.msi INSTALLDIR=c:\projects\server SERVICENAME=mariadb ALLOWREMOTEROOTACCESS=true /qn
   - cmd: "\"c:\\projects\\server\\bin\\mysql.exe\" -e \"create database testc\" --user=root"
+  - cmd: set MARIADB_CC_TEST=1
   - cmd: set MYSQL_TEST_USER=root
   - cmd: set MYSQL_TEST_HOST=127.0.0.1
   - cmd: set MYSQL_TEST_PASSWD=

travis.sh

@@ -52,6 +52,7 @@ else
 
   if [ "$TRAVIS_OS_NAME" = "windows" ] ; then
     echo "build from windows"
+    set MARIADB_CC_TEST=1
     set MYSQL_TEST_DB=testc
     set MYSQL_TEST_TLS=%TEST_REQUIRE_TLS%
     set MYSQL_TEST_USER=%TEST_DB_USER%
@@ -62,6 +63,7 @@ else
     cmake --build . --config RelWithDebInfo
   else
     echo "build from linux"
+    export MARIADB_CC_TEST=1
     export MYSQL_TEST_USER=$TEST_DB_USER
     export MYSQL_TEST_HOST=$TEST_DB_HOST
     export MYSQL_TEST_PASSWD=$TEST_DB_PASSWORD

unittest/libmariadb/my_test.h

@@ -556,6 +556,13 @@ static int reset_connection(MYSQL *mysql) {
 void get_envvars() {
   char  *envvar;
 
+  if (!getenv("MYSQLTEST_VARDIR") &&
+      !getenv("MARIADB_CC_TEST"))
+  {
+    skip_all("Tests skipped.\nFor running unittest suite outside of MariaDB server tests,\nplease specify MARIADB_CC_TEST environment variable.");
+    exit(0);
+  }
+
   if (getenv("TRAVIS_JOB_ID"))
     travis_test= 1;