CI: Fix more test failures

• CI: Remove meson-dist and cmake-dist build systems

  Before we removed the Autotools build system, these meant: do a make dist with Autotools, then unpack the resulting tarball, and build it with Meson or CMake (as appropriate). This was important because the contents of an Autotools make dist differed significantly from what's in git: only the files that were explicitly declared as sources in the Autotools build system were distributed, and Autotools would also add its own generated cruft.

  Now that we're doing releases with Meson, the official source release artifact is basically git archive, so there's much less need to distinguish between official source releases and what's in git, and these variant build systems became simple aliases for meson and cmake. Remove those aliases from tools/ci-build.sh, and stop using them in .gitlab-ci.yml.

• CI: Don't re-run the tests when preparing to build as a subproject

  After running the test suite, we don't need (or want) to re-run the test suite as a side-effect of producing a dist tarball. Historically, this test-case used Autotools make dist, not make distcheck, but when we removed the Autotools build system, it was replaced with meson dist (similar to Autotools make distcheck). Change that to meson dist --no-tests, which is more like Autotools make dist as originally intended.

  In particular, when we re-ran the test suite, we weren't filtering out the time-consuming and timeout-prone tests that emulate malloc() failures, which we don't normally intend to run in CI.

  Resolves: #489 (closed)

• CI: Set language environment variables to safe values

  Recent versions of glibc support C.UTF-8 natively, and both Debian and openSUSE (which we use for our CI) have been patching it into our glibc versions for several years before that.

  Helps: #488 (closed)

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In fact this fully Resolves: #488 (closed) as I had hoped.

/cc @rhabacker @pwithnall